Also keep in mind that since their last SB win in '07 Belichick has broken down and re-built his defense entirely, with the exception Wilfork. So, it's not a shock that his defense would struggle. Heck, that last SB team in '07 bears very little resemblance to recent teams with only the aforementioned Wilfork, Mankins, Welker, Ghostowski and Brady being the only leftovers.
For most any team that rebuilds as such, they end up being terrible. The Pats, OTOH, since '07 have won 65 games against just 20 losses. This during a re-build!!!!!!!!!!!!!!!