The more I think about it, the more I am convinced its about a reduced role AND money.
I have suspicion that the Rams did indeed make Jackson aware of his reduced role in 2013 and wanted to pay him as such. The market value of a backup running back.
If Jackson doesn't advertise himself as a featured running back, he doesn't make featured running back money.
You know how the saying goes about athletes who say its not about the money... It always is.
** This is not a knock on Jackson and I wish him the best wherever he may go. I have nothing but respect for Jackson and the Rams organization how this handled this very difficult situation.