The problem with gauging a team's pass rush by raw sack totals is that it doesn't just measure a team's pass rush, or even just its pass defense -- it's actually primarily a measure of how many times over the season a team found itself in a high-sack-probability situation.
The most obvious factor is that if a team is good enough in other areas to be frequently ahead by more than a score in the 4th quarter, they're going to see more pass attempts than other teams, and feel more free to "pin their ears back," as they say.
A teams "pace" also comes into it -- depending on both offensive and defensive styles of play, some teams just pack more plays into a game than others. The San Diego defense face a league low 162 opposing drivers. The Eagles and Cardinals were both tied for 3rd most, with 193. All three were playoff teams, but the SD defense had over 30 fewer drives in which to get sacks.
And then there are ton of even harder-to-pinpoint situational factors that play a major role in sack totals. For a variety of reasons, including shear chance, some defenses end the year having faced a greater proportion of 3rd and longs than others, thus having better sack opportunities.
Another huge factor that rarely gets considered is the team's opponents. In the NFL, outside of one's division, teams have very different schedules. Teams like the Pats and Ravens have only two common opponents all regular season. Sometimes you get a schedule packed with Roethlisburgers, Warners, and whoever's playing behind Buffalo's o-line, and sometimes you get Mannings, Bradys, and lots of short-passing offenses. In the NFL, any statistic that doesn't try to adjust for opponent's faced is going to be relatively useless.
