There's a big disconnect between STEM subjects and liberal arts in my experience. As an engineer, I was used to classes where exam averages were in the 60s (30's does seem kind of low), but I learned to only care about the curve and where I stood on the distribution.
Contrast this with liberal arts courses where generally an 85 on an exam meant a B and a 93 or 94+ meant an A. It's fair enough to say "this paper about social dynamics in medieval Italy is a B quality paper". It's harder to say "this is a 55/100" and here's how it stacks up against the curve in the way that many engineering and math courses allow. At the end of the day though, this translates to inflated grades in liberal arts as compared to math.
The other issue is that (pure) math at the university level is just harder. I suspect that many people who are good at number theory can probably wing it in a literature course. I doubt that vice versa is true. This isn't to downplay liberal arts degrees... the fact that you didn't have the toughest major doesn't make you less intelligent nor does it mean that the field is less important. In fact, in some cases it's the opposite. I know many people who did "tough" majors only to regret it when liberal arts folks with higher GPAs had easier times getting accepted to grad schools.