from my favorite stat site and he makes the same point
Top Offenses > Top Defenses
In terms of SR and EPA, the best offenses are "better" than the best defenses in terms of performance. And in terms of WPA, the best offenses have bigger impacts on game outcomes than the best defenses do. This is something I wrote about three years ago, when I noticed that the distribution of yards-per-play efficiency was wider for offenses than for defenses. Now, in terms of more advanced statistical measures in a broader set of data, the same trend holds.
A variable's standard deviation (SD) is a measure of the width of its distribution. The ratio of the SD of offensive SR to the SD of defensive SR is 1.25. The ratio for EPA is 1.27. And the ratio for WPA is 1.26. Offenses are spread out 25% wider than defenses in terms of performance and impact on outcomes.
The reason, I suspect, is that most of the offense flows throw the QB or RB. The QB in particular is singularly critical to offensive success. The QB is responsible for much more than just throwing passes. He calls audibles, reads the defense, calls blocking assignments, and is responsible for organizing and managing the offense. Nearly every play an offense makes is heavily dependent on the skill of a single player.
On defense, success depends on a more equal division of responsibility. The wider the division of responsibility, the more "average" the success of the squad. In individual-player sports such as tennis or golf, individuals can dominate the field for many years. Federer or Woods did not have their talent level diluted by less dominant teammates. A five-man bball team can be "star-based" because there are only four other players to dilute the skill of the star.
http://www.advancednflstats.com/2011/01/top-offenses-top-defenses.html
He has a glossary if you don't know what EPA is and he explains what is and the math involved.
Last season Denver & San Francisco had two of the top defenses in the NFL and Baltimore who was below average defeated them both.
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