Nope.  Argument Clinic was merged in 3.4, and in 3.3 pwd.getpwuid wouldn't accept strings.  So this isn't a bug introduced in the Clinic conversion in 3.4, this is historical behavior, and we can't change it now.

If anything, I'd prefer that grp.getgrid *didn't* accept strings, but it's too late to change that too.  "Helpfully" automatically calling int() on an argument is un-pythonic.

(To answer your specific questions: they probably shouldn't have different behaviors, and I assume there's no particular reason for the difference.  It's probably that they had different implementors, and one did a sloppier job than the other.)