Now, that being said, there is still a potentially large gap between what you might see on the field if you were to watch every play of every game and what UZR “says” happened on the field. And that is one of several reasons why one year or even 10 years of UZR (or any other sample metric) does not give us a perfect estimate of a player’s true talent or even an accurate picture of what actually happened on the field. The reason for that is that the data is imperfect. For example, UZR might put a certain batted ball in a certain bucket and determine that that batted ball was extremely difficult for the CF’er to catch, based on the recorded (by the BIS “stringers”) qualities of the ball and other data. We don’t, of course, know for sure whether it was indeed a difficult to field batted ball. We don’t know exactly where each fielder was stationed, we certainly don’t know the exact location of the batted ball to the nearest square inch on the field, and we definitely don’t know how long the ball was in the air or on the ground.
http://www.fangraphs.com/blogs/the-fangraphs-uzr-primer/