Again, the following observations are help others put your reported experience into context:
What is "non-routine"
If you were issued ANY additional request, including those in a CIT 0520, that is considered "non-routine." Which in itself does not mean much at all.
In particular, "non-routine" is merely a descriptive distinction for applications which involve ANY procedures or requests that are in addition to those that all applications go through. It is not a formal status, so there is no label on a file that says the application is "non-routine." A mere fingerprint request, which is fairly common, means the application is "non-routine," meaning it has involved a request in addition to the standard procedures applicable to all applications.
I cannot explain what the officer meant or why in your case. Your experience appears to vary from the norm some.
In particular, few applicants actually have conversations with the Citizenship Officer who evaluates and makes the final decision in their case. It is more common for applicants to be interviewed by a processing agent who gathers information, adds comments or such to the file, and submits the file to the Citizenship Officer. Sometimes at test events Citizenship Officers conduct some of the interviews, along with the processing agents, in effect sharing the interviewing workload. I cannot say how it is decided which applicants are interviewed by a Citizenship Officer versus a processing agent. But it occurs to me that perhaps the applicants have been sorted and, possibly, it is flagged applications which are handled by the Citizenship Officer.
There are Citizenship Officer interviews apart from the standard or routine test event interviews. These are essentially an informal hearing conducted like an interview. These are not at all common. Some are about residency but they can be an oral test of knowledge of Canada, or a test of language ability, or to address other issues such as prohibitions.
Labeling CIT 0520 as a type of RQ:
Here too, the "RQ" or "RQ-related" or "RQ-lite" labeling is informal. The formal RQ, that is the "Residence Questionnaire," is CIT 0171. It is specifically labeled "RESIDENCE QUESTIONNAIRE." For more than five or six years now, CIC then IRCC has been making RQ- related (as in related to residence or presence questions) requests that are much smaller in scope than the CIT 0171. The most common version of these RQ-related requests is the CIT 0520. Very early on (more than five or six years ago or so), many began referring to the CIT 0520 as RQ-lite as a shorthand reference distinguishing it from both the full-blown RQ, that is CIT 0171, but also distinguishing it from some other forms of RQ-related requests, ranging from quality assurance requests like the CIT 0205 many applicants were issued in 2017 (which actually requested more than a full-blown RQ) to specific requests in simple letter form (but nonetheless requesting residence or presence related information or supporting documentation).
These and more are discussed at length and in some depth in a topic specifically about the various forms of RQ-related requests; see
https://www.canadavisa.com/canada-immigration-discussion-board/threads/rq-versus-physical-presence-questionnaires-including-cit-0205.534082/
Here, for example, is a quote from a post of mine there last summer:
Mostly for reference, here is a quote from 2017:
In any event, while I have not seen the most recent version of the CIT 0520, for years these too had a heading which specifically referenced being a "Request For Documentary Evidence of Your Residence in Canada" (with some slight variations depending on the particular version). Whether the current version references "Residence" or "Presence," the CIT 0520 is for sure a non-routine request (most applicants do not get these) for RQ-related information or documentation. However one otherwise *labels* it, these are requests for residency or presence related information.
As I recall, in previous posts you similarly referred to the CIT 0520 you received as "RQ light." Nonetheless, again the label is unimportant.
What Triggered Issuing CIT 0520 in your case:
As I previously noted, your experience differs from the norm some because you had a conversation with an Officer rather than simply the typical processing agent conducting the test event interview.
But even if the interview is with a Citizenship Officer, the interviewer comments tend to be superficial and at best gloss over or skirt any real issues. They are trained to not give away much.
Apart from that, however, it is telling that in your interview you were questioned some about the unreported (in the presence calculation travel history) trip to the U.S., more thoroughly than just asking for clarification that it was a day or landing-flagpole trip. Again, isolated mistakes, including omissions, rarely trigger concerns or questions. Again, even during the period of time CIC was applying draconian triage criteria sweeping up to one-in-three applicants into RQ, a single isolated mistake in travel history was not triggering RQ.
The underlying question is why, why in your case, was this omission a focus of inquiry? Given the record of your landing date, as mistakes go, it is not as if failing to report the flagpole event was concealing the fact that you entered Canada at a PoE on that date to do the landing.
I do NOT KNOW why. It is very, very difficult to figure these things out. Other circumstances likely factored into why there were enough concerns to question you more extensively about this. The "other circumstances" that factor into these sorts of decisions can be entirely innocent. Ranging from details in the address and employment history, to your travel history other than this trip, as well as what other indications of ties to the U.S., or not, among many others.
Of course details like the margin over the minimum requirement often factor into this (noting, however, that even applicants with many months of presence over the minimum have gotten RQ, including full blown RQ).
Moreover, it appears that the decision to issue the request for additional documents was made attendant or after the interview. Which means how you responded to questions was a factor in the decision-making. Lots of room for variability in outcomes when a total stranger bureaucrat is assessing the credibility of an applicant's oral responses to questions.
OVERVIEW:
Again, I have gone into the weeds some regarding this to better illuminate the process, recognizing there is more than a little we do not know, so that
@Klamm and others can have some context.
A key aspect of this is to remember that ANY applicant can be issued RQ-related requests, and even the full-blown CIT 0171 RQ. Mere coincidences can trigger RQ (like having a postal code in the address history that is in an area where addresses have been previously used in cases of suspected fraud).
Thus, for example, in reference to the query posed by
@Klamm in particular, my observation that the failure to include the flagpole trip in the calculator travel history should not, not in itself, trigger much if any concern or any RQ-related non-routine processing, that does not mean I am assuring
@Klamm there will not be any questions or RQ-related non-routine processing. But of course this isolated factor will be among the full range of things considered by those officials (processing agents and Citizenship Officers) who make the decisions which determine how it goes.