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.