I once read an article that explained a lot to me. We all have noticed that we can wear different sizes depending on brand or cut. Well, the official reason for that is that there is actually no sizing template for women's clothes. Basically, the companies really do just make up sizes. In fact, some companies will take what would normally be considered a size 8 (sometimes even 10), and label them a 6 - for marketing purposes.
Studies showed that women, when given the choice to buy a 6 (that may be more expensive and/or not as flattering a cut) or a size 8-10 that may be cheaper and more flattering of a cut, they tend to buy the smaller size anyway. (I am so guilty of that).
They really should have a universal sizing template so we could at least know and accept the size we are (or do something about it). It is very upsetting to wear a size, and think that you are that size, only to find out that you can't wear anything else in that size, you know?
Oh, and I totally agree w/ Calcium that they don't make clothes for women's bodies. Why is it so hard to find a pair of jeans that fit over hips/thighs w/out making a big pooch over my butt. Hello, women tend to have waists that are smaller than their hips! Why in the world do they make jeans with almost the same sized waists as the hips so it creates a big pooch.