In the game those when the player is created it is determined whether or not he has the potential to be a star. This is actually not that far off of real life, when a person is born he has a certain amount of postential to do some things as he grows through life and he really has no control over that max potential.
That said, I was thinking and although I haven't tested this yet I am pretty sure I have found another way around some of the potential rating and maybe even somewhat a way to mold a player's progression. Lets say for example you have an 85 overall RG who is at his potential. Right before the offseason if you change this player's position to either RT or LT it will probably drop his overall but leave his potential the same. He will then progress as though he was below his potential. You can then move him back to RG and his overall will go back up with the position change up to above where his potential was setting his potential rating at this new level. There are other positions where this could be done as well such as DTs to DE, DEs to LB, some LBs moving from outside to middle or vise versa, some DBs being moved around, WR and TE or RB etc.