I have been present in a departmental review of sshrc apps, and basically the people who are going to make the decisions go through each of the files, then meet to rank the applications. The research proposal is one of the most important considerations, but academic history (as determined by transcripts and the GPA calculation that the administrator makes for each file) is also important. Adjudicators may also speculate as to who they think will be more likely to be successful in the other competitions (ie. I prefer this app, but don't think she'll be as successful as the other app). The files are ranked (1,2,3,4,5) and submitted to Grad Studies for another review. The process is not "scientific" and any numbers associated with ranking are relative.
When my Masters application was forwarded on to the national competition I asked people (that is, university admins) what the stats were, they said that once an app is forwarded to the national competition the chances are about 50/50. I still have my fingers crossed but the good thing is that UofT has a pretty good track record... but knowing that people are already receiving notifications is making me a little jittery...