Why are some girls thin and they don't even exercise?

Well, I'm 13 and there are heaps of girls and my school and they are so thin and I don't mean thin in an anorexic way, I mean like their body is so perfect and toned! I mean, I know and you can tell that many of them don't even exercise because there are many girls like this in my PE cl and seriously... they don't even make an effort to participate for like the whole lesson (most just sit at whatever chance they get)! Then I see them at recess and there eating chips/ice-cream/chocolate/lollies, etc. Other than genetics is there any reason why this is? Are they just born that way?