We all know Ibanez had trouble running in on balls. But what isn't mentioned often is that he's pretty good going back on fly balls to the wall and warning track. I had a conversation about this subject with The Fielding Bible author John Dewan, inventor of the Plus/Minus defensive stats system. Dewan told me Ibanez was a much better fielder than he got credit for. Not a Gold Glover, but far better than the guy he replaced in Philly, Pat Burrell.
The reason was Ibanez's ability to go back on balls.
Now, in Philadelphia, he's got less ground to cover in left field. So, I'll take a stab at why his UZR is so much better. I think the ballpark in Philadelphia plays to his strengths. If you know he can cover ground running back to the wall and you know there is less distance to cover in left field there compared to at Safeco Field, it makes sense that Ibanez could be cheating further up towards the infield.
By doing so, he can cut down the number of blooped hits that drop in front of him and penalize him on the UZR front. At the same time, he can still track down balls he runs back on because there is less ground to cover before he gets to the wall. And, he has the skills to make the tougher plays when he's running back
I'm not a sabr-head by any means so correct me if I'm wrong, but doesn't UZR account for where the player is positioned? Bakers' explanation seems to explain why he might be putting up better numbers in the +/- system rather than UZR. Could be wrong, though-- I'm asking here because I'm not sure and I know you guys can set me straight.