Another way to look at it: the 13-point teams allow 21 or fewer points 64% of the time, while the 14-point teams allow 21 or fewer points 49% of the time.

It seems like the distribution of points allowed shouldn’t be that much different for a team that scores 13 or 14 points, but it is. It works out to something like a 160-game difference. And that would make sense. If a team is trailing 28-7, they are much less likely to settle for field goals in the second half. Instead, they score the TD to get them to 14 points. Teams that are at 13 points (or get to 13 points) more likely got there because the game was close and attempting those field goals did more to increase win probability.