Sabermetricians have a favorite statistic when attempting to predict which MLB teams will improve or worsen significantly from one season to the next: win-loss record in one-run games. The prevailing opinion is that the outcomes of such games, over the course of a season, are primarily decided by luck. A bad team can get a bloop single to break a 2-2 tie in the ninth and win by one; a good team can groove a fastball to the wrong hitter with the game on the line.
Really good baseball teams tend to win a lot of games by multiple runs; really bad teams lose a lot of lopsided outcomes.