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I believe it was Bill James himself who said he would expect defense to be more variable than offense. There are all kinds of minor little tweaks that cut into range that would have little to no effect on offense. If Branyan were limited to light jogging do you think he wouldn't still be at 1st base? It's different with a prime defensive position of course, but I would still expect that a Guti that can only run 80% because he tweaked a groin muscle would still be in the field.