Underdog has the connotation of being significantly undermanned against a superior team. a matchup of a 12 and 13 is not a situation where either team is an underdog.

Here's the Oxford dictionary definition: "A competitor thought to have little chance of winning a fight or contest." So not just expected to lose, but to not even have a realistic chance.

In NCAA terms a 16 seed vs a 1 seed is an underdog situation. A 7 vs a 10 is not.