Team run differential is the total number of runs a team scores in a season minus the total number of runs a team allows in a season. Team run differential is a useful statistic that can be used to measure what teams have been either lucky or unlucky. For example, teams that have a small run differential but a very high winning percentage are likely to not win as many of their games over the rest of the season. Chances are such a team has won a high percentage of close games, and even though some believe that this would be the result of an excellent bullpen or an inherent ability to win close games, research has shown that teams that win a lot of low scoring games are no more likely to win such games in the future.
More about Team Run Differential
It is used in the Pythagorean Expectation which was invented by renowned baseball statistician, historian, and writer Bill James. Baseball run differential helps identify the quality (or luckiness) of a team's wins.