You're wrong. They even explicitly call out that it's not about weight. Everyone has different proportions.

> Once I compared my personalized sloper to commercial patterns and retail garments, I had a revelation: clothes were never made to fit bodies like mine. It didn’t matter how much weight I gained or lost, whether I contorted my body or tried to buy my way into styles that “flatter” my silhouette, there was no chance that clothes would ever fit perfectly on their own. Finally I understood why.

Later they show how J.Crew has a certain ration between hip/waist for all sizes. Even if you had the same sized waist, chances are you wouldn't have the same sized hips as they expect. Most bodies just aren't that shape.