It has to do with the different point values players get for defense.
Item AIII A-2.0 Fielding
Fielding was the toughest one to mimic as players may be open to multiple positions. In the end, we kept eligibility at 20 games played in this season and last season for any one position. Defensive weights had to be different per position though.
Catchers
Error, E: -32
Games Played, GP: 4.8
Corner Infielders
Assist, A: 4.48
E: -32
Putout, PO: 0.48
Middle Infielders
A: 1.28
E: -32
PO: 2.4
Outfielders
A: 6.4
E: -32
GP: 4.8
There are no defensive points awarded to players in the Utility position or for pitchers.