Perhaps this image will help explain what I mean:
So, you see, my current first baseman does have a rating in left field and center field, but not in right field.
However, his base defensive ratings in the outfield are 5 (out of 10) outfield range, 5 outfield error, and 6 outfield arm.
So I could conceivably play him in right field and as he gained some experience there he would then be rated at that position as well. But his potential to become very good at that position is very limited. He has average range and only a slightly above average arm. If he played right field every day for four or five months he would probably become an adequate right fielder.
But if his base defensive ratings for the outfield were instead 2 (range) 2 (errors) and 2 (arm) then no matter how much I played him there he would never become acceptable at that position.
Is this more clear?