The gut feeling many of us have is that BYU isn't really performing well against legitimate opponents. When you dial into the stats, this is exactly true when you compare Mendenhall's tenure with his contemporaries we talk about so much.
http://www.cougarboard.com/board/message.html?id=14209280
Winning 70+% of your games doesn't really mean as much when it was from a softer Mountain West schedule. It's a great stat, but doesn't tell the real story when you compare BYU to other programs, and especially consistently ranked and relevant national programs.