[DonsDiscountGas]

I was surprised to read that too. I think the answer is that here we're looking at admissions rate = number admitted / number applied, which is not the same as overall difficulty in a conceptual sense.

The only people applying to grad school in math are people who got a BS in math and did so with good grades (or perhaps some other STEM field + significant theoretical math coursework). On the arts side I suspect they draw from a larger pool (plus people tend to switch from STEM to something else a lot more than the other way around) of backgrounds. It's easier to convince oneself that a short story is great (when others may disagree) than convincing oneself a math proof is correct when it objectively is not. So there's less self-selection on the applicant side, and hence a lower admissions rate.