I believe there is a direct correlation between the AL and NL central having the best records in both leagues:
The Cardinals, Pirates, Cubs, and Royals have good clubs. That's obvious. But the 2 divisions also have some terrible teams and faced off vs each other in Interleague play.
Milwaukee, Cincinnati, the White Sox, the Tigers, and the Indians all have bad baseball teams this season. This really allowed the upper echelon teams to fatten up on the bad teams.
The Royals built such a massive lead that they lost their edge to some extent. The month of September has basically been a formality. The Cubs, Pirates, and Cards have benefited from being in a pennant Chase.
On a side note, I still think the Mets are the most dangerous team in the NL.