Betting games especially sports betting can really surprise you because even though the game's odd are all laid off and there's already a predicted winner, tables might turn near the end of the game. It's not something someone can really tell because there's always a slightest chance of winning to those behind.
The underdog might be able to take home the title without anyone really expecting. It's a mix of luck and strategy for the team and a matter of possessing the right knowledge and odds favor for you. It will depend on how you will assess and analyze their record and the game itself and how the team will lift themselves to make it to their goal.
I don't think there is luck here. It's just that when an outsider wins, it's always a surprise, because before that, in similar games, the favorite won many times. But the favorite is not strong enough to win 10 games out of 10 or 100 out of 100. Therefore, draws and defeats of the favorite are natural (when they happen rarely). I think if we transfer the analogy to chess, then any grandmaster has a chance to beat Carlsen in one of 10 games, but for example, I dont have a single chance even in 1000 games. It's a matter of balance of power. When the favorite wins, it is natural, but it cannot always win, because outsiders can do something too.
There are people who do really believed that ML or Moneyline do always win which they arent really that putting up into their minds that odds or chances is always there if we do talk about upset.
Yes, we can really say that it should really be a sure win but the bad behavior of ours as a gambler is that we do really love to have that all-in bets which is never been that recommendable.
If you dont really like for yourself to get into a situation where you do make yourself fucked up then its better always consider on having that bet which is something that you could
afford to lose or something which your emotion could bare into because if not then you would really be stressing out yourself that much or the worst you would really be in rage.