I understand what you are saying, thank you. Let me be more specific in what i have experienced over the past few years on occasion (including pre-Alamo website 'enhancement').
I typically book a SWA flight that will arrive late am/early afternoon on a Saturday and return the car in the evening the following Saturday, resulting in an 8 'day' rental i.e. a weekly + 1 day rental.
I've noticed when i'm considering taking the crack of dawn flight down on a Saturday that when i price out the rental car picking it up @ 8 amish on Saturday the weekly & daily rate has often substantially decreased from my original, later arrival time.
Note that I attempt to price the res out various ways, including: 'mock' modification of original reservation, also clear cookies, use different web browser, even have tried in private browsing...but i find i cannot replicate the earlier pick-up's lower price on my existing reservation arriving early afternoon same day.
I've called Alamo in the past and questioned 'why'. I was told it's a matter of projected capacity/availability of vehicles at certain times and that unless my flight was delayed they wouldn't honor the reservation past a two hour window.
I don't dispute what you are saying, just sharing what I was told. I only called once, maybe I should've called back ala Disney to see if I received a different response? Anyway, I'd be thrilled to be able to book an earlier pickup time the same day (for the same rental period) and save a few $ if you tell me MCO will honor the rate.