Player Ratings Drastically Changing?

I have a question about player rating changing from season to season:
For example I'm starting as a low level coach at TN-Martin and I brought in a 2 star freshman. When he actually joined the roster he was a 3 star with 4 star potential as a freshman. The next season he was a 4 star sophomore and played well. 2nd best player on the team behind my 4 star PF that was already on the roster when I started the game. Then for his junior year he drops to a 2 and a half star with 2 and a half star potential. And this has happened with a few others after my 4 years with the team. I just dont get how the potential and actual overall fluctuates. Am I missing something?
For example I'm starting as a low level coach at TN-Martin and I brought in a 2 star freshman. When he actually joined the roster he was a 3 star with 4 star potential as a freshman. The next season he was a 4 star sophomore and played well. 2nd best player on the team behind my 4 star PF that was already on the roster when I started the game. Then for his junior year he drops to a 2 and a half star with 2 and a half star potential. And this has happened with a few others after my 4 years with the team. I just dont get how the potential and actual overall fluctuates. Am I missing something?