I cannot tell you any reason why an A- is better off than an A, if I understand the game, the advice you are getting is wrong.
I can tell you that a reason in addition to baseline prestige that you are an A- and Mich State is an A, might be something similar to baseline, and that is where both of your prestiges were at 5 seasons ago (i.e. your prestige I am guessing was worse 5 seasons ago, hence although you may have almost closed the gap, you still have not caught up to mich state)