OK. I was short and skinny, and no good at basketball. Some kids are born with a physique which made them superior players, and they put in no more time and effort than I.
The same is true of my AP Calculus students (I'm a former engineer who chooses to teach college-level math to high school kids). Some kids are born mathematicians and take to vector calculus like ducks to water. Others - despite quality instruction (I know who their algebra teachers were!) - cannot seem to master algebra. Any kid who thinks sqrt(a^2+b^2)=a+b will fail my calculus class, and there's not much he or I can do about it.