It's really not.
The one stat that really matters is the final score. Average PPG on a sample of 16 games does not mean much. One single game, good or bad, will skew the average.
The 2011 defense, which couldn't stop a nose bleed, was 15th in the league in PPG (21.4). The 2009 defense was 5th (17.8 PPG). In 2011 we got to the Super Bowl, and in 2009 the defense was run over by the Ravens in the wildcard game.
Again, I'm not saying this defense is awful. I'm not saying it's great either. It has flaws, which can be exploited by good, efficient offenses. Belichick and Patricia can overcome some deficiencies during the regular season, because most games are played against mediocre teams. But at some point in the playoffs, we will face a good, efficient passing team, and if the offense is sub par, I don't believe this defense has shown it can overcome that.
When I read all these comments about how great the defense is because of their regular season PPG average, I feel like a Colts fan. We are celebrating something that is not important and has no bearing on our playoff chances.