I don't know this for sure but I am assuming those numbers are based on what teams have done in the past. If it says 100% it probably means that no NFL team has ever made the playoffs at 3-7 behind 4 1/2 games...
It has nothing to do with historical probabilities.
It's based on what the teams have done this year. As it explains on the page that they run every game the rest of the season and simulate the results based on team performance this year. After 10,000 simulations, they give the results.
It should be remembered that there is rounding involved. So although it says 100.0%, that doesn't mean we make the playoffs in all 10,000 simulations. We could fail to make the playoffs in as many as 4 or 5 seasons (depending on which direction they round) and it would still list us at 100.0%
But it should also be remembered that football outsiders thinks that STL, SEA, and SF are really bad teams.
Since each of those teams can't lose more than 2 games without elimination, it's likely that the simulator merely thinks that none of those teams can win 2 games while we simultaneously lose all our games (remembering that Football outsiders rates us as the 6th best team in the league).