I was thinking the same thing. I also measure my hips across the widest part of the butt and  get 39.5" every time. I'm 5'6" and 138 lbs. That puts me at a bmi of 22.3 - by no means skinny, but pretty average. I don't look disfigured, but I obviously have some noticeable junk in the trunk. Size 8-10 jeans fit my ass and hips, but my waist SWIMS in them. It sucks.