@JakeFrost -
Yes, there are stipulations - most likely the one that affected you was the "availability" :
an upgrade on compact through full-size car class bookings (based on availability).
Cars have been affected by the pandemic and this is continuing because of now related recovery issues. Rental companies sold off lots of cars during the pandemic just to try and survive (some didn't) - now when they need more cars because of the recovery and people want to travel - there is a big shortage of cars - new and used New cars are being affected by a chip shortage. Rental companies as well as other sellers, have tried getting cars via the used market but there are shortage problems there too in good used cars on the newer side - used car prices rose 7% in the last quarter.
FORBES 03/11/2021 - A Car Rental Apocalypse Is Coming: Here’s What You Need To Know
Maybe with time, all of these things will even out but it might take awhile - Do things actually go down after they have gone up - well sometimes when supply and demand shifts the other way.
There is a new player in town called Turo - I have just read about it - kind of an Airbnb-type service for the rental car area that lets private owners(host) rent out their own vehicles.
It's Always Something . . . . Roseanna Roseannadanna