Math PhD student here. It seems to me that mathematical ability is a nebulous concept. I've noticed in courses I taught that grades tend to reward conscientious students who can "play the game" and do formal manipulations even if they don't really understand what's going on. Courses tend to move fast enough that very few students can keep up with all the concepts, so the ones who have trouble playing the game and don't keep up with all the concepts have trouble.

Personally, I had little patience for that. I seldom memorized formulas. Either I knew them from repeated use or I just derived them on an exam. I always felt odd when I had to apply a technique or formula I didn't understand. I would say that love of learning seems to have played a significant role. When I truly want to learn something and take pleasure in doing so, I'll devour the subject. I have trouble making progress in topics I feel forced to learn.