Could it be the speed of the turf that is not reflected? Mientkiewicz fielded a lot fewer balls than did Giambi, but is clearly a better firstbaseman. The stats and PB reflect that reality. But the home turf will allow Giambi a lot more playable balls -- balls that in Minn would have been in RF before he moved.

FYI, Rivas groundball stats at 2B are terrible too.

I think probably the original complaint here is that PB does not factor home park into fielding like it does hitting/pitching. I don't know what stats are available, but it seems that the math could be done.