Alaska373890 and oftech pretty well nailed it on the head. I'll add that for every combination of make/model/trim/mileage at resale, the rental car companies know, almost down to the penny, how much profit they'll make off of each vehicle before they even purchase it...
unit cost - depreciation + rental revenue + resale price
Typically they load up on the trim levels that will be the most profitable in the end - less profitable trim levels are usually manufacturer leases that get returned to the manufacturer while the car is still "young" by rental standards.