Were most of our good numbers against teams without much of a rushing offense?
Best way to compare apples to apples is ask how the teams did against us compared to how they did overall. Here's how it stacks up on a yards-per-carry basis (note that I'm using Super Mario's numbers for the game average and ESPN's final stats for the season average).
Games we allowed a team to significantly beat its own average:
at Tennessee (9.6 ypc game versus a 5.2 season average)
Jacksonville (5.78 game; 4.5 season)
at Buffalo (4.85 game; 4.4 season)
at Indy (4.0 game; 3.5 season)
Indy (4.95 game; 3.5 season)
Games where opponents basically hit their average:
NYJets (4.52 game; 4.5 season)
Games where we held team well below its average:
Oakland (2.14 game; 4.1 season)
at Arizona (2.75 game; 4.1 season)
at Cincy (2.7 game; 4.1 season)
SF (3.1 game; 4.3 season)
Tennessee (4.95 game; 5.2 season)
at Jax (2.97 game; 4.5 season)
Seattle (2.58 game; 4.0 season)
at St. Louis (2.58 game; 4.3 season)
at Miami (3.75 game; 4.4 season)
New England (3.89 game; 4.1 season)
As has been noted
ad nauseum, our defense in weeks 4-17 was radically different than the sad-sacks that took the field in the first three games. Of the six games where we allowed an opponent to match or beat its yearly ypc average, three were the first three weeks. After that, the only teams that had more than their usual success running the ball against us were Indy and Buffalo. Buffalo's total was bolstered by TO's 30 yard TD run on a reverse that Dunta was just too sexy to defend. And Indy was one of the worst rushing offense in the league, but I'll ascribe our struggles against them to Peyton's hex over us.
Everybody else found the going on the ground tougher than usual when they played the Texans (and, in most cases, it was seriously tougher). Of the 10 teams that we held well under their season average, 6 were in the NFL's top 12 in terms of overall yards per carry (Tenn #1, Jax #6, Buffalo #9, Miami #10, St. Louis #11, San Francisco #12). So we were putting the clamps on all comers after week 3 unless they were QB'd by Peyton Manning.