Strength of schedule is calculated from the opponents projected fantasy points allowed for the DEF 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.
The Green Bay Packers are a great example of the 'disconnect' that sometimes exists between the NFL and fantasy football. Sure, the Packers have a decent defense (15th in yards allowed, 13th in scoring defense in 2014), but it isn't much more than that. Yet the Packers were a top-five fantasy defense in most scoring systems last season? The reason for that gap? Dom Capers' 3-4 zone blitz is a very aggressive defensive scheme, and while the Packers give up yardage in chunks from time to time the defense also annually does well in the sack and takeaway departments.