That's pretty strange since there is such a big discrepancy, but it could still be that you just have very dense bones. I'm pretty sure that that's a good thing, so be happy! Also, having a lot of muscle can make you weigh more even though you look lean, but it wouldn't make you weigh that much more. So I think it's a combination of the two.

Don't worry about it!

Muscle weighs much more than fat so if you are really well toned and don't have much fat that might be why you are heavier than you want to be. Remember that a size 8 is a good size