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 defensive line has been playing better as of late and the offensive line of the Chargers aren't world beaters by any stretch. Rivers has done a solid job protecting the ball and escaping pressure for the most part though. Giants' DEF makes for a top ten option but they are much closer to 10 than to 1.
New York suffered horrifically with injuries last year, at one point fielding almost an entire secondary of second teamers. Decimated would cover it. This was compounded by an out-of-sorts season from Jason Pierre-Paul that helped drag the G-men from third in sacks in 2011 down to twenty second in 2012. Off all the teams outside the top 5 this is one of two defensive teams that might have the potential to exceed their current rankings.