I've seen this happen and it's not the rider. (My bottom is quite symmetrical, thank you.) Since we make a lot of leather products I can assure you that it is very difficult to catch all the odd hides that stretch more, less, or unevenly; it drives me nuts sometimes. There are simply saddles out there made from bum leather (pun intended) by both Brooks and Ideale.
Chris
http://velo-orange.blogspot.com/
Chris Kulczycki
Velo Orange
109 Colonial Avenue
Annapolis, MD 21401