In the NL, if Arizona and Chicago split their doubleheader and both Cincinnati (against Los Angeles) and San Diego (against Houston) lose OR if Arizona sweeps and Cincinnati and San Diego win, then all three teams will be either: 55-53 or 56-52.
Under the current tiebreaking rule, there would be playoffs to decide all the spots. In the past, the Reds would have been automatically in the playoffs as the division tiebreaker between San Diego and Arizona would have taken preference. Then somebody must have thought, "Why should a team that finished in second place get an advantage over a team that finished first?"
So, now the loser of the divisional playoff would play the team out of the division to decide the wild card. Yes, it wouldn't add up right overall, but it would likely be more fair.
Meanwhile, in the AL East, the Yankees play Toronto while Boston faces Cleveland with the Yankees needing a win to maintain their .002 lead in the division. Although if the Yankees and Red Sox both win, the Yankees lead would increase to .003 with rounding (.60952 to .60747)