Some of these classes don't seem to make any sense at all.
The Corolla and Ion are much smaller than the Optima, and smaller than many of the "compact" cars. If you pass up a "compact" Ford Focus to get an "upgrade" to an "intermediate" and find a Saturn Ion there, that is not really an upgrade (the Focus is bigger inside and better).
Somehow, I doubt the drivers who would like to drive a BMW 325i or 525i would be satisfied with a Kia Amanti or Toyota Avalon. Or vice-versa.
I think this is their way to lure customers to reserve a car with them and charge an upgrade later. I recently reserved a Mazda6 (or similar), but when I picked up my car, the agent told me I can get a Nissan Sentra or Toyota Corolla, or pay extra to upgrade to Altima. Despite the fact that there were several Mazda6 on the lot, the agent claimed that they were not available because they didn't show up on her computer.