They are #1 in yards/games, but they are #31 in attempts against/game. When your pass defense is suspect teams aren't going to run on you all day...
Further, the only top ten runner they've faced (total yards) is Tiki Barber, and he had 15 carries (for 60 yards) against them.
Chargers built a reputation based on last year's performance. Last year they got their record pretty much the same way the Bills got theirs: by beating a bunch of non-playoff teams. You do have to give them credit as they were able to beat all the non-playoff teams they faced (unlike the Bills last year), but when it came to facing teams that made the playoffs, they had one victory, and that was against a divisional foe.
This year they are looking to repeat that pattern. Of course that means with us being a "non-playoff" team they should beat us. But I have trouble figuring out what all the hoopla is about them sometimes too...