I'd assume it's a measure of both attentiveness and intelligence. And also of how much reading you did when young. I expect all these things correlate enough to make it hard to disentangle them, but just on first principles it seems obvious that you'll learn spellings better (1) if you generally learn things better, (2) if you're exposed to more correct spellings, and (3) if you're paying more attention to spelling relative to other things.

I agree about the difficulties of spelling reform. Perhaps sufficient support from high-status intellectual literary people might get past the "reform looks like ignorance" problem. Strong support from George Bernard Shaw wasn't enough for English in the early 20th century; perhaps it could be done with a large enough coalition of obviously expert people, or incrementally with each smaller step perhaps being easier to accept.

(Whether it would be a good idea, I don't know. I've not seen evidence that the difficulty of spelling in English -- which I think is one of the hardest-to-spell major languages -- causes much actual harm. And yes, for the avoidance of doubt, the Mark Twain thing [EDITED to add: very probably not actually written by Mark Twain] I linked to was written as a joke and not a serious proposal; I linked to it because I think it's funny.)