Recently, I took a research about why there are difference processing times between immigration applications of IRCC.
One of the most possible reason is that the security screening processing time are quite different, it is one of the step of background check.
So please do not argue IRCC's employee, they are just the normal people like you and me, I think the management system in IRCC could be the most possible have some issue, need to be improved.
Below information come from the article I found, it helps me, and I wish it could help you all.
if you want to know more details, please go to this address :
All about Security Screening
(
https://gcmsnotes.com/all-about-security-screening/)