PHILADELPHIA—The Seattle Seahawks were facing a 40-year-old backup quarterback who had never taken a postseason snap and was unexpectedly thrust into action in the middle of a playoff game. The game should not have been close.

But somehow, with minutes left, Josh McCown had the Philadelphia Eagles within striking distance of Seattle in their wild-card game on Sunday. It was acutely on brand—for the Seahawks. Their games are outliers because they’re so statistically tense and nutty.

...