Defensive measures tend to swing around wildly. The problem is probably that Harper had a few bad games defensively and hasn't yet had enough good games to make up for them. By the end of the season he almost certainly will though.
Same deal with Dickerson, just the other way around. Dickerson isn't a good fielder. He's probably made a few spectacular plays early this season, and his usual lackluster plays haven't made up for them yet. But they will.
Think about it this way: Jed Lowrie isn't going to lead the majors in RBIs this season. But he is leading the majors in RBIs right now. He's had unusual success batting with men on base early this season, but by the end of the year his batting in clutch situations will drop off and Springer or somebody like that will lead the league in RBIs. Same deal with Harper/Dickerson on defense.
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