This is an issue of "can lose" vs actually losing.
Upsets happen. That seems to be the concerted point being made by all of my besties here. No shit. But each individual upset is an unlikely event.
Like being struck by lightning. Or getting in a car accident. Or your school winning a NC. Each of these happens, but when it happens to you, it's surprising because it's unlikely.
If we wanted to go further, we could say getting in a car accident is like a 10-point underdog winning. Your school winning a NC (if it's a top-15 all-time program) is like an 18-point dog winning, and being struck by lightning is like a 30-point dog winning.
The point is that "something that happens sometimes" is different than "that something happened this time" or "that something happened to me."
Again, no matter how many people have the time to kill to pile on, this is a truly obscure point to try to disagree with me on. I'm not wrong. A 21-point underdog team, no matter how talented, surprised everyone by beating Nebraska that day. Obviously.