If you excel at math, you are doing it better than anyone else. You might even be moving so much faster than the rest of your class, you're put into a special, accelerated math program.
Excel derives from the Latin excellere, which is all about going beyond the high. Someone who excels at the violin might practice five hours a day because they are striving for excellence, meaning outstanding-ness. You could describe their performance as excellent, or fantastic. That doesn't mean you yourself want to practice five hours a day. Excel means to stand out, and some of us are pretty happy standing right where we are.