All girls put on a layer of fat on the lower body area during puberty. (Yeah, life sucks.) Genetics plays a big part in it; fat cells can be shrunk/depleted, but they do not go away. (See part above about life sucking.)
My problem is different; my thighs are strong tho not perfect. I have good arm definition for an amateur weightlifter. My calves, on the other hand, are enormous. They have always been larger than average (strong, yes, but large) even when I was young and very lean. I'm doing unweighted calf raises at high reps. And to think all the guys at the Y complain about their skinny calves ... too bad we can't trade!