What I noticed this morning...

I noticed this morning that some people have flat butts. I mean really flat. These people gain width as they get older, or fatter, or at least seem to, but there just isn't much volume back there. Why is that? It makes me wonder how they walk, or climb stairs...I really think that is what the muscle back there does... Anyhow, that is my quandary for today. How is it that some people--notably some women--have flat butts?