I think it comes down to enjoyment. I studied Computer Science at Uni, and there was a clear dividing line between those who struggled and those who did well.
Those who struggled were the individuals who took the subject primarily because they wanted a high paying job/wanted to impress family, etc. Those who did well are the ones who did it because they enjoyed the subject.
When the "career" guys got an assignment they struggled with, they panicked/gave up/tried to get somebody else to do it. When the "fun" guys got an assignment they struggled with, they found it fun, you would see them discussing solutions with friends - not "copying" or anything, it was a competition to see who came up with the best solution.
Same is true back at school. The kids who tried their best to get out of homework, found lessons boring, etc, ended up falling down to the bottom sets. The kids who actually did and enjoyed the homework, participated in lessons, ended up climbing to the top.
Now, what causes some people to enjoy certain things and disregard others, I wouldn't know. It's hard to argue either nature or nurture, considering that siblings often have very different attitudes from each other on all manner of things.
EDIT: And the same is true regarding sports. I remember disliking sports from a very young age, and so I never really got good at them. Other kids loved sports, and ended up playing for local teams as they grew older.