I bought this art book recently to get into figure drawing. Im training on drawing the mannequin torso and hips but the hips are fucking me up bad kinda cause the author does an absolute shit job at breaking down how to do it.

I snapped this photo so you can have a visual of what I'm looking at for a reference. The one on the second page, bottom left, is what's really fucking me up. I thought the hips were supposed to be a trapezoid but that's a fucking rectangle. Can someone help me to understand this?