Why did I understand the concept of a Riemann Sum integral when I was 10, but had problems with fractions?

In the sense, I understood how phones estimated the amplitude of the sound wave to convert it to a digital signal, somewhat, but it took me a long time to understand how to divide fractions (oddly enough I could multiply them fine). Similar things in earlier high school classes. I understood summation notation immediately, but it took me a while to understand how to use the algebraic formulas. In AP stats, I understood the complicated topics, but I'd always screw up with the easy things.WHY AM I GOOD AT THE HARD STUFF BUT FAIL AT THE EASY STUFF??? O_O It makes no logical sense!