Once a baseball leaves the pitcher’s hand, he has little control over what happens next. The batter could turn a 100-mph fastball in on his hands into a bloop single. He could blister a 110-mph line drive right at the left-fielder. Luck has always played a role in baseball, but now there are more ways to quantify it.
Sometimes, a pitcher will get extraordinarily lucky for an extended period. They may walk too many hitters and give up a ton of hard contact but maintain a strong ERA thanks to a low batting average on balls in play (BABIP).