I'm very slim with a high body fat ratio?

Hi, I'm 5"2' and 8cm, and I weigh around 7st2/5 (it goes between those ran ges). I'm 16 at the moment, but I have a body fat ratio of 30%! I don't know what to do, or what I'm doing wrong, because I do lots of exercise. I run and I do quite intense cardio sport twice a week. I eat a pretty balanced diet, I eat chocolate at the weekends and stuff but I didn't think that was bad!! I eat lots of salad/potato/pasta etc so I don't know how to fix this?!