It's not just the playoffs either.
According to Football Outsiders' playoff odds report, the Bengals have an 76.1 percent chance of winning the division, a 34.2 percent chance of capturing a first-round bye, and 30.3 percent of securing the second seed. Based on their (sometimes hard-to-understand) formulas, the Broncos are set with the first-round bye, followed by the Bengals, Colts, Chiefs and either the Ravens, Titans or Jets.
How do they figure this stuff out?
Instead of trying to translate, their explanation.
The playoff odds report plays out the season 50,000 times. A random draw assigns each team a win or loss for each game. The probability that a team will be given a win is based on an equation which considers the current DAVE ratings of the two teams as well as home-field advantage. (DVOA ratings are explained here.) Mean Wins is the average number of wins for this team over the 50,000 simulations. Odds are given for winning each playoff seed, as well as for winning the division (DIV), earning a bye week (BYE), or winning a wild card (WC). TOT represents odds of making the playoffs in any fashion.
I guess they're just have to play the games.