I would surmise that it has something to do with the likelihood of a winning team (let alone a team that is 6 or 7 games over .500) being outscored by 29 runs is fairly low.
Pythag is a tool. I don't necessarily love it and think it has some real holes (namely accounting for blowouts) but that's still a pretty extreme result and you could make an argument that AZ has been the luckiest team in baseball this year...if you put stock in Pythag.
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