Why do so many people get degreees in math and physics when they do so poorly in their exams?

>hurr durr I got 40%
>yay, since I was top of the class it's basically 100% after the curve

Is it because the subjects are inherently more difficult than those in pretty much any other major so we have to throw in some fudge factor so that people can get their degrees?
I've often felt like a faliure getting 60% out of a set of proofs and I see many people here that would probably be ecstatic, why?