For more info on the stats used by Football Outsiders, you need to read this.
Sometimes the story simply writes itself. The Indianapolis Colts -- long an offense-first team with a history of early playoff flameouts, save one Super Bowl win three years ago -- reach the Super Bowl again after their new head coach jettisons the longtime defensive coordinator to bring in a more diverse and aggressive scheme. The players fill up reporters' notebooks with quotes about how they are finally attacking opposing offenses rather than passively playing their previous coach's antiquated Tampa 2. Thus, the defense must be better. The problem is that, objectively, the Colts' defense under first-year coordinator Larry Coyer is no better than it has been in recent seasons.