An average SS makes around 80 plays outside of his zone.
An average 3B makes around 40 plays outside of his zone.
This makes sense, since a SS covers twice the ground a 3B covers.
So, if a SS has 25% more range than the average SS, he will get 20 more plays out of his zone.
If a 3B has the same 25% more range than the average 3B, then he will get 10 more plays out of his zone.
So on top of the positional adjustment, the SS who has the same skill level as the 3B, will get a UZR range rating 10 points higher.
I'm not even saying this has to be a problem. But I do think it's something we should be aware of when we compare players.