Actually, before the end of the game, we can only predict the outcome of the game but we cannot say with 100% certainty which team will win. However, I think that in the case of sports betting, analytical results can contribute a lot to winning. That is, if there is a game between a weak team and a strong team, if someone bets on the side of the strong team, then the chances of winning are high, but sometimes the strong team can lose the game, in which case the gamblers suffer. This is mainly their bad luck. And in other gambling cases, the outcome depends entirely on luck.
The simplest explanation is that gamble is determined by luck regardless of whether it is sport betting and the likes... Gambling as we already know is a game of luck... And so we just can't really say for certain how a certain bet or a match would turn out. In the case of the strong team winning the smaller team that you talked about, well that isn't always the case... As majority of times the underdog have can of given certain spectacular performances which by far has even outclassed that of the bigger teams..