The "best position" is actually a "suggested position" and can change based on training. It is actually calculated based on the player skills, primary taking into account relative skills.
It is quite possible that one of your players whose "best position" is SG will be your best performer as a PG.
For example:
Player 1: Really good shooter, Not so good passing and ball handling.
Player 2: Good shooter, OK passing and ball handling.
Player 3: Can't hit barn door with shot, but is OK passing and ball handling.
Players 1 and 2 will have a best position as SG, Player 3 as a PG. You might be better using Player 1 as your SG, and Player 2 as your PG, and use Player 3 as a backup (or selling him or firing him). You will get just as good passing and ball handling from Player 2 as Player 3, and better shooting. PG's shoot, but perhaps less than SG.