You're placing far too much importance on something that is meaningless. The playing position displayed is simply a suggestion based on their stats. It's there mainly to help new starters have a general idea of who they need where and what skills those positions generally need. But as you get more experienced with the game you might want different stats at certain positions. For instance I play a very outside focussed offence so it's more important to me that my point guard can shoot than pass so i play a "shooting guard" in my point guard spot.
If you train a player then eventually they will change their displayed position. For instance this season my "power forward" changed to a "center", but that certainly hasn't stopped me playing him at the power forward spot.
So in short - just ignore it. Play and train which ever player you think best at the position you think best in the line up.