Sizes are not consistent between brands, either. I can wear a 16 in some brands, but an 18 is tight in others. It's even the same in kids' clothes. There are brands where my daughter can't get into an 8, but others where an 8 is even a little loose.
Body shape make a big difference too. I'm an apple when it comes to body shape, so when a pair if slacks fits me in the waist, they are usually baggy in the butt and thighs.