against Alabama's schedule. By subtraction, this means that an avg FBS team would have a W/L record of 2.1-3.9 (essentially 2-4).
With BYU being 6-0, an avg FBS team would have 3.1 fewer wins, meaning a W/L record of 2.9-3.1 (essentially 3-3).
Alabama has played a tougher schedule but both have overperformed by a similar number of wins.