For a few reasons:
1). It's a division game. We know them and they know us. It can't be argued that the Jets have more talent then Miami AT THIS POINT. That could change but it won't happen before Sunday. We are the better team.
2). Revis returns (hopefully). The Dolphins don't have scary receivers, so Rex and Pettine will dial up more blitzes this game to confuse the neophyte QB Tannehill. But the D will need to maintain it's run discipline to keep Bush in check.
3). The schedule. In the real world, teams peek ahead to the next opponents on the schedule. We have the #1 and #2 teams in the NFL coming to our house in the next two weeks. To a man, each Jet knows that Miami is a must win. Beat Miami and lose to SF and Houston and we're 2-3, still in contention, since no one will run away with the AFC East. But lose to Miami and the next two as well, we are on a 4 game losing streak looking forward to 2013.
4). The Sparano Factor. This is the biggest factor IMHO. To atone for last weeks debacle, and the fact that their OC is returning against his old team, the offense will be a highly functioning machine. I'm talking about 40 points, 400 yards of offense with Mark/Tim relatively clean. They want to give Sparano a game ball Sunday night.