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Teixeira, Gardner, Swisher, Cano and Jeter (yes Jeter) were all above average fielders in '09...Jeter seems to have finally gotten annoyed enough with people ripping on his glove that he's working harder on ranging to his left and you don't see as many balls go past a diving Jeter into center field...whether that's permanent I don't know.
But in 2009, their DER was high because they got a career year out of Jeter with the glove and solid contributions at several other positions...there are probably other factors as well...UZR seems to think the Yankees were below average defensively (by 19 runs)...so there is a suggestion that the team had good pitching that was inducing a lot of flyballs and weak contact and that was helping the DER.