Strength of schedule is calculated from the opponents projected fantasy points allowed for the DL position. The best rating is +16 which gives the player the advantage against the defense and indicates an easy opponent, the worst rating is -16 which gives the defense the advantage and indicates a tough opponent.
Williams has toiled in relative obscurity in Buffalo, one of the better defensive tackles in the NFL that most people have never heard of. This year, Williams will switch to defensive end in the 3-4 front of new defensive coordinator Mike Pettine. That's bad news for Williams' IDP prospects in 2013, unless your fantasy provider still gives Williams positional eligibility at DT.