What is UP with sizes?

As I was digging through my closet this morning trying to decide what to wear, I realized that I have 3 different sizes of clothing in my closet...and they ALL fit me. So, obviously, it would be nice if they made sizes universal, although I have also realized that I like some manufacturers better because of things. At the Gap, I wear a size 6, so they are definitely a well-liked company. Come on ladies, what mother of 2 doesn't like to still wear a 6. Smart thinking by the Gap, because I will spend more money. However, at Express, I wear a size 10...not as good. So, considering how silly we women are...who do you think will get more money from me!?! Hmmm...