I know, you don't wana hear that, In the beginning, i was also thinking the same, how come?
But reality is
Every file is different. Because of many reasons listed below
1. When you applied Which year? if you applied in 2016, your case may take maximum 3 years. My uncle aunt got PR in 14 Months. On the other hand, I had applied in 2010. Still waiting for PR to be stamped. Many people applied in 2011 already got landed and some are waiting. However new rules and esp new Govt after 2015 case are being processed faster
2. How you show your income, with T4 or self employed
3. Number of dependents in PR application. CIC take time to check education certs etc..
4. In which department Principal applicant (at PR file) was working. Background check, If it is Military or Police may take longer time as compared to person, not doing anything. For e.g. house wife
5. Travel history for PR applicant. If PR applicant traveled in another countries i think it takes longer to check
All above i have learned by reading post from other users here and from my experience for my application. Apart from that, I have no authenticated resource/information for all above
Hope that help
Thanks