I'm not sure I buy that. Obviously seated people are half the height, so half the variance, but that assumes the upper body range difference is the same as the bottom.

I had three reasons, but they were just guesses, I'm not convinced. One is based on the idea there is a cycling type who in performance male version has long legs and smaller upper body, lots of power to push, but no more upper body weight than necesarry. Two, the idea is that the tubes are fixed length so they run out of materials in the longer sizes and there are constraints, but since tubes are available in a variety sizes, seems unlikely. 3, is the idea that there are certain markers that buyers are looking for like a wheel base length or frame weight, and the longer sizes get squeezed. If a big guy can get onto the shorter top tube length, maybe he wants the more nimble wheel base.