The NFL is set up so teams in the same division have the same schedule (or 14 of 16 (87.5%)) similar games at least, as they currently have two games based on where you finished in the division the year before. (which will be 14 of 17 if/when they go to 17 games).
While teams from other divisions might only share 4 of 16 (25%) games (4 of 17 in the future).
So the NFL is saying the teams that share that 87.5% of the same schedule should have one team that host in the playoffs, over a team with a better record that might only share 25% of the same schedule.
Weather we like it or not... that system is fair... as if you come in 1st in your division, you get the spot. If you don't come 1st, you don't get the spot.
What if the NFL only allowed division winner into the playoffs? Would it still be unfair? No, because it's not unfair to start with, win your division, and you get the spot... it's that simple.
Now, I have disagreed with the NFL leaving out teams, but that's a different argument. I have dreamed of a system (when there were only playing 16 games and only had 6 teams per conference make the playoffs), that they should have a flexablity playoff system, where any team that won 10 games was automatically in the playoffs, even if they were the 7th seed... and those years when the 7th team won 10 games, the #2 seed would be forced to play wildcard weekend when normally they would be.
Or simply a system where all division winners and 10 win teams (62.5% wins in a 16 game season) make the playoffs.
10 wins in a 17 game season would be a bit more than 58.8% wins.