The Pats are to some extent enjoying smoke and mirrors similar to the Dolphins' success this year, in that they've surrendered an opposing passer rating of 103.8 (the Dolphins 102.9), which is third-worst in the league and has been associated with a 4-12 regular season record on average since 2004. The loss of Hightower doesn't stand to help that any, of course.
The difference with the Patriots, however, is that their offensive passer rating is 108.3, which is also uncannily high and explains their winning record. Tom Brady is in essence overcoming their defensive ineptitude.
The Dolphins on the other hand are suffering through an offensive passer rating of 78.1, and so the fact that they have a record similar to New England's is truly remarkable.