That really has to do with the competency of the scouts who both scouted Brady and Russell. Belichick def saw some talent in Brady so he drafted him, and fortunately for him he got an All-pro, superbowl winning quarterback. Brady had potential to Belichick and probably exceeded those expectations. Russell def did have high physical potential, but there are other facets than just the physical to a quarterback's game and that def limited his potential.
I agree that potential is something that can't change, however people's perceptions of talent are obviously different. Some might say well Russell was an A potential type player, or some might say no his mentality isn't all there, he should be a C potential type player. Really to me, each player should have a set potential, but the potential should change based on the scout your team has. If your team has an A+ scout then he should be able to accurately predict your player's potential. Also the system you play in should also affect how quickly or if your player's reach their potential. Right now, Peyton Hillis is playing to his potential because he is familiar and comfortable in the system. The system suits his skill set, but to say his potential should change is wrong, he is just playing near his max right now. Also reaching potential and how quickly should also work on your coach. If your coach is a great teacher, yes than said player should reach his potential and play at it.
Changing potentials shoudl be based off of the perception and skill of scouts not off the play of a player, who might just be playing up to his max potential. The current system is fine where it is, but adding in the scout, system, and coach features and making them more involved would add greater realism.