Schools offer $ as opposed to % because cost of attendance (COA) changes (increases). They have to report % to NCAA. And let's assume they promise you that dollar amount for all four years. The % of COA will decrease for the 2nd, 3rd, and 4th years due to COA increases. That leave more % for incoming players for those other three years.
And perhaps they only offer it for one year. Then I would imagine the coach would offer the same number in subsequent years (unless he lowers the amount or takes it away based on performance) because he can say he's sticking to the original offer--but of course the % is less--which is all that matters to the NCAA.