I don't know why you can't score A's at math though. It makes no sense to me as to why you can help others get good grades but can't get good grades yourself. It probably means you lack the hours applying through repetitive practice.
Similarly in chemistry lab, I could help anyone else but I would invariably run all the right tests on all the wrong samples. This made me a great lab partner but a disaster on my own.
As I have gotten older I have learned coping mechanisms but some things I can't do and I pass those onto others.
At the same time I have always been top of my class in many things, while putting in quite a bit less effort than average. These ranged from history to some of the sciences, to some cases of math (calculus, some algebra).
There are some areas where I am quite frankly talented. Once I get Perl coding on accounting software, there is very little that can stop me and quite frankly I can outperform nearly any programmer I have ever worked with.
On the other hand, there are also things which I cannot do very well no matter how hard I try. When I was in college for example I used to be able to help struggling students in areas like analytical geometry understand the problems and go from getting C's to getting A's. But I couldn't get above a C myself.