Yes, I noticed this too. There is no set pattern at all.
Nov 2 AOR here and my file is untouched since I cleared medicals on Nov 30.
Whereas many folks from Dec AOR are getting RPRF request, while I am still waiting to receive one.
I called IRCC yesterday and they said my application has been pushed to June 11, 2019 for completion and eligibility, criminality and security have not started

This just sucks