Not a perfect theory that they had overshoot the target! This is a government plan and the immigration ministry is bound to answer the parliament about any deviation from the actual target. What if mejority of the issued ITAs were successful, how they would have accommodated those extra PR in the targeted 73700? We did not hear from any government or non government resources that they have gone over the target! And I do not believe they purposely kept some applicant awaiting untill 2018 just because they could not accommodate them in 2017, that would be a major lie from Canadian government. Being one of the most transparent countries in the world, I am sure they haven't done something like this.
We still do not know for sure what are they considering as their PR target - is it successful landing or is it approval of PR application?
Unless the immigrant lands he/she is not considered as a PR. I got my PR approved in 2017 but I landed and became PR in 2018 so would I be counted in the target for 2017 or 2018?
For easier tracking of the targets, they may be considering approval of PR application as the condition to meet the target. So if that is the case, then ITAs issued in the later part of 2017, don't get considered in the 2017 quota instead they get considered in the 2018 quota if they get approved in 2018. So there are a lot of variables and we cannot assume anything based on the number of ITAs they issued in a particular period.
And they need not keep any applications waiting, because there could be applicants who received ITA in Oct 2017 or later who have not even submitted their applications to CIC till 2018.
CIC would always do some analysis based on the past data they have before arriving at the number of ITAs being issued as can be noted with the fact that they change it time and again.
Once the report for 2017 is release, we would know more accurate details. Till then, all we can do is assume based on what trends we see right now.
Also, having higher number of ITAs towards the later part of the year would ensure that they do not overshoot targets and also to ensure the system remains competitive.
For example, if CIC issues 60,000 ITAs in 1 shot on Jan 1st, the cut off would be in the 300's but if they do the same by splitting the 60,000 throughout the year, the cut off can be retained in the 400's as more number of people would be applying throughout the year.