No matter how sure you are, once you risk your money on it, it’s still gambling, even if you’re 99% or 99.9% confident. Unexpected events, like injuries during the game, can instantly drop your chances of winning from 99% to 50%, or even flip the outcome completely. Even without any unfortunate events, there are times when the underdog performs well and the favorite ends up having an unlucky or bad game and that's nothing new in sports.