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NickBid McPhee was selected for the Hall of Fame long after most of the others. I expect that there are collectors who segregated their HOFers from the rest of the Old Judges and never updated their collection by moving the McPhee into the other category, but who subsequently graded all their OJ HOFers (meaning that their McPhee is sitting raw). I've seen the same thing happen to an extent with T-206s, as George Davis cards were not submitted in large numbers prior to his election. [Of course, the fact McPhee was not a HOFer for a long time also might create more of a condition bias, as people did not take as much care of his cards.]
Managers might be less frequently graded because they're not players (less people trying to build a complete graded set of HOF managers), but that would not account for a disparity in graded numbers between managers.