What departments are you applying for? Some are, generally, rich, while others are less rich. I've heard of stipends varying up to $10k across departments and schools
The way I understand it is that it's fairly well adjusted for the cost of living wherever the school's located. The stipend from UCLA is around 35k, but Dartmouth's is only ~25k. So yeah, around $10k variance. I'm going to accrue some dept, because Ramen noodles are going to get really boring after a couple months. That's ok though, this is an investment in future salaries. What's ironic is that I'll be making far less during my post-doc than I am right now, but things should rapidly improve from there.
Honestly, I think it's just cultural... a nationwide pact between universities to milk every last drop of cheap labor out of soon-to-be ex-students. If I ever hear otherwise, I'll try to remember to let you know.