I've heard the argument before that the measurements are based on averages, but honestly? It just doesn't make any sense.
At Old Navy I'm pretty solidly a small, if not bordering on XS, at the moment I have three pairs of Calvin Klein slacks that fit in a size 6 and my Levi's size 8's are too big so at least in those brands I'm a size 6 pants with a XS-S shirt. However, if you look at my measurements I'm
Bust: ~40 (depends on the bra I'm wearing so if anything it could be slightly larger)
Waist: 31.5"
Hips: 40"
So at Old Navy, for instance, according to their size chart I should wear an L or something between a 12-14 on top (according to their size chart a Small should fit someone with a bust between: 34-35" and a waist between 26-27"). I'm sorry but that would have to be some AMAZING stretching for that to make sense if the clothes were really cut for a woman that small.
I have some medium shirts/dress from them and while I don't have a picture of me in them, I could take one and you could see that they are absolutely massive on me, no way could I ever fit into a large!!!!
According to Calvin Klein's size chart a size 6 should have: Waist 27" and hips 37.5" Um... far from what I have!
I don't feel I wear my clothes super tight AT ALL, actually those size 8's from Levi's are very baggy on me at the moment and are seriously saggy in the butt!
So I figure it could be a few things:
1. manufacturers are lazy and haven't updated their size charts to recent changes they've made in their fit models.
2. they'd rather have women err on ordering too big sizes so that they feel good about themselves. Also if they return the clothes they might even buy MORE clothes in the process because they are so happy
3. they don't think anybody actually checks the size charts so they don't care
4. they'd rather draw people into the stores than have them ordering online
Whatever it is, it makes me really mad. I don't have the luxury of going in store to buy clothes being that I live abroad so I've had it happen to me more than once that clothes were too big after ordering off of their size chart.