No, it's not random. If you look closely, people from July, 2012 onwards are getting PRs in the order the applications were received by CIC. First July, then August, then September and so forth. Another interesting thing to note is the ECAS status of these applicants. It's working just as it is meant to work.
So, in my opinion, July 2012 onwards the processing is not random. On the contrary, it's planned, systematic and most importantly fast.
And of course not 100% of the applications will follow the pattern. There are numerous factors why some will not make it in time. It is part and parcel of any process. And we are talking about CIC here! So we know.