Yeah, there's that too. The warnings signs were there with Peavy well before he moved to the AL. There's a definitely a ton of pitchers who show that they're clearly benefiting from pitching in the NL as opposed to the AL, but between his elbow issues and his home and road splits in much of his latter career with the Padres, Peavy likely wouldn't fare too much better in the NL for long. And Willis was getting shellacked in the NL, too, hence his trade to the Tigers in the first place.