Strength of schedule is calculated by averaging the rating average of all FBS opponents. Apologies for the last couple of weeks and early this afternoon. I had inadvertently posted the weighted average for opponents rather than the overall average.
I'd also like to take this opportunity to note that the opponents' average includes the game played against that opponent. So if Alabama beats LSU, they won't get credit for beating a team with LSU's current rating, they would only get credit for what LSU's rating falls to after the loss. I know it would be better to factor those games out, but I just don't have time to calculate all of that. Anyway, as a result, if an undefeated team is a few spots below a winless team, chances are the undefeated team actually played better teams. Those teams just look worse given the extra loss.