In some cases where RCMP tracing report releases something unexpected, IO would require PCC for Canada from applicants.
These are my assumption only:
When a file is ready for police clearance, IO input name info on their computer system which is linked to RCMP database. If the name come up as 'no tracable results found', taha, you're done with RCMP clearance checks. If your background does not trigger anything significant, they wouldn't bother requesting background security checks from CSIS. You are done, completely done with background. If your name is unique and you are cleared, you will be likely fallen into this "luckier" group!
Scenario #2, IO input your data, system doesn't say either you are cleared or not cleared because there is similar name with criminal record come up. System recommends IO to request RCMP screening by fingerprint to clear. Raj may fall in to this scenario.
Scenario #3 (the worst one), IO wants to conduct both police clearance (with police) and security clearance (with CSIS, FBI, intelligence agents) before they can clear you. You will never know how and how long this process is going to take. But, only under certain circumstances - to certain applicants - will an IO conduct this kind of screening. This is worse than the worst in terms of processing time because this is out of CIC control.
Note that every case undergone different levels of screening. Why Ottawa can process cases within 4-5 months, maybe because minimal screening files are transferred to Ottawa so they don't have to do much before request PP. One of my relative whose file was tx to Ottawa but then they found out she was missing PCC from her country, they immediately tx the file back to Buffalo and advise her all further processing will be done by Buffalo and she is still waiting now to hear from Buffalo after sending in her PCC. My observation is that Ottawa only handles easy and straight forward cases.