But that's pretty much the point. The NL is a weaker league (offensively) than the AL, when you try to compare ERA+ from player to player with such varying degrees of difficulty in the division that they toil in, then unless the NL player absolutely sucks, the NL-AL factor will always present him with a slight advantage, Arroyo being a case in point.
Don't get me wrong, he's a decent pitcher, but no way you get away with a 9.9 H/9 in the AL East and sustain and ERA in the Mid-4's. Bill James said it best: "In the AL, the DH alone constitutes a .50 difference in runs scored per game", not to mention the bottom of NL lineups as compared to the bottom of AL lineups. It's a vastly different league, and while i'm not saying ERA+ is a flawed stat (I like it) i usually tend away from using it to compare players who toil in divisions as different as the AL East and NL Central.
Answer me this: If you were a pitcher, and you had to choose a division to pitch in to pad your stats, where would you pitch?