I know this isn't "ask the dedelman." But (a) the NL is the weaker league in Fingers, and (b) if MON is resting him consistently against tough LHP and you were resting him randomly or he was having minor injuries to account for his missed games, that could contribute to the difference.
In a desperate effort to stay with the theme of the thread-- Mike, referring this back to another current thread about ratings vs. stats, is this player a good example of why even for major league hitters, stats may be deceptive, whereas ratings never lie?