It might be somewhat of an eye-opener, but you don't just go to 13-3.
We got close to beating Dallas, we shut-out Minnesota, We beat Washington, we beat Philly... We even beat the Seahawks in the play-offs by quite a margin and we came close to beating the SB champion NYG while they where already on their hot streak.
I know where this thread is going, but I'm going to blame this on the decline of the O-line and D-line. Where our D-line was our absolute strength on defense last year, it's the biggest liability this year and the O-line is obviously declining. Quarterbacking was very good in both cases.