my player Geerts was Power foward, but now he is center...why????
The "best position" is more a "suggested position" and is calculated based on a player's current skills.
How he performs in a game is based on his actual skills and the position you play him, and not his "suggested position". That is the game engine will see that he has average inside defense, respectable rebounding, inept inside shooting and that he is set as a C or PF in your lineup, and
not that he is a "power forward" or "center".
The skills needed for a C and PF are similar. The main difference is that PF will have a pretty good jump shot. So before, you had a center with a jump shot better than most centers, and now you have a power forward with somewhat better inside skills than other power forwards.
Note that in basketball, it is common to refer to the positions as 1, 2, 3, 4, and 5 rather than PG, SG, SF, PF, and C. So and so is a "4". So now it is like saying that Geerts is either a "4" or a "5".