Maybe (like airlines) they are getting better at their fleet and not having excess cars sitting, which in turn yields sold-out situations more often.

I've seen a few smaller locations have less cars than typical, but it's not across the board. One other thing I've noticed is that at a lot of locations, I try to book a 1-2 day rental, as most of my stays in a city are short. But Avis shows sold-out. If I bump it up to a 4-7 day rental, they magically have all types of cars available. So I book it longer, and return early. Whenever this has been the case, the lots are flooded with vehicles when I pickup and return. Seems odd they're giving up the revenue for short rentals when they could just raise the rates and capture some of the business.