the real cutoff isn't at the end of -rc1, Linus frequently pulls things in from Andrew in -rc2, and sometimes puts patches that he things likely to conflict with other changes in -rc1 so that people get a cheap bisect-like test to see which one causes the problem
it's usually around -rc3 that the changes really stop.
the other thing is that not all regressions get documented as such.
documented regressions are usually when a person finds the problem and needs to report it for others to research. if the developer finds the problem they just send a patch and explain why it's a regression, they don't waste their time to submit a regression report.