3.) The Bills received help down the stretch from other teams
When the Bills lost to the Philadelphia Eagles in Week 12, they fell to 6-6 and there was doubt that was placed on them as far as making a run at the playoffs. The AFC was tough and the Bills could not pull off a big win against what was the number one team in the NFC at that time.
They were the 11th seed and needed to play perfect football the rest of the season, along with receiving some help from teams. From Week 13 and on, it could not have gone any more perfect for the Bills. They won their last 5 games of the regular season and received help from teams who had defeated AFC contenders that owned the head-to-head tiebreaker on the Bills (Jaguars, Broncos, and Bengals).
In addition, the Dolphins, who were at the top of the AFC East from Week 5 up until Week 18, fell apart the last month of football and they lost the division to the Bills in the final game of the regular season, losing 21-14. Both Buffalo and Miami finished at 11-6, but because the Bills swept the Dolphins this year, Buffalo won the tiebreaker.