Variability exists, but we live in a deterministic universe, it's explainable in theory and extrinsic to behavior. So a guy goes out to a restaurant the night before a day game and has a slight case of intestinal discomfort. The next day his performance is adversely affected. Several factors may or may not account for his performance and how much it deviates from typical performance, none of which depend on luck. We may not be able to quantify the contributions of all the variables with any certainty because of how complex the interplay is between all of them. That's what makes sports fun.
Variability of behavior and luck are often used interchangeably by people.