My experiences with math and science are somewhere between the_inev's and jl's. I think I was bad at math because I really didn't care about it one way or another. I saw what the teacher was getting at, then didn't bother to practice it enough to make the leap from "getting it" to "being able to do it with any level of skill". Science, on the other hand, was totally, deeply fascinating to me, and I was always getting in over my head because I'd take science classes that required more math proficiency than I actually had.
Also, the_inev's experience might have had to do with some sort of brain development that finally got to where it needs to be. Young adolescents really aren't playing with an adult deck in terms of cognitive ability. This is a reason why one-size-fits-all curricula fail for a chunk of the population (and why you can be good at one class and suck at another - over and above interest and effort).
My dichotomy came to a head in college, where I ran into walls in chemistry and physics because I'd blown off trig in high school (I hated proofs). Faced with the prospect of failing in classes I really wanted to learn, I suddenly became Math Nerd and took 3 semesters of calculus. This was ultimately a bad idea as well, as biologists have little call for much beyond simple integrals and derivatives, while statistics are very, very, very useful. I learn stats in my spare time now.
**adjusts invisible horn-rims**