Also, if you look at their run D this year they really haven't played anyone. If they're still #1 in a month I'll be impressed. Right now they have the benefit of an easy schedule in their corner.
i already posted in another thread about their run defense being suspect.
on top of not facing a very good rb yet this season, they've had to defend the fewest rushing attempts in the league because opponents are forced to abandon the run and play catch up.
for what it's worth, pff has them with a neg run defense grade.