That only sorts schedule toughness by oppoents winning percentage!
See WVU - played no difficult non-confernce games and plays in the worse BSC conference. Their oppenants winning percentage is high b/c it includes Louisville and Rutgers who also built up nice win-loss records by doing the same.
ND played 4 Big 10, 3 Pac-10 and 2 ACC teams. So just like any other BSC school they played at least 9 games against BSC conference opponets. One of their non-BSC conference opponents was a bowl team.