Because humans are not very symmetrical. You probably have one kidney that works better than the other, and are very likely to be much more flexible or strong on one side than the other.
Shoes are not made identical sizes left to right (except for the really cheap ones made in 3rd world countries, because they don't know or care), and neither are well-made bra cups. Women's left breasts (and feet) tend to be about 1/2 a size larger on average, where for men it tends to be the right foot --which is another reason why women often have trouble wearing men's shoes.
Happily, people don't expect symmetry when they're looking so you're probably the only one who notices.
at 9:10 PM on Feb. 23, 2011