In sports, matches against closely ranked players are hard to predict.

Take Sindhu vs. Okuhara for example. As of Aug 3, Sindhu trails 5-6 against Okuhara. I believe Sindhu won a game after that making the score 6-6.

Clearly, in this case, the winner is hard to predict. It depends on various factors: performance in that game, physical fitness (how tired a player is), the mental makeup on that day (tough games require a player to constantly motivate themselves and fight back), what one ate (an upset stomach can cause issues), etc.

Practice can improve overall performance and make the performance on a given day more predictable. Practice can also improve physical fitness.

In sports, there are too many unknowns to make it hard to predict a winner. If it is hard for individual sports like badminton, imagine predicting a winner in team sports like soccer or cricket when the teams are closely matched.