Cars are rented out with the assumption that prior renters will return the cars on time. If that doesn't happen, you will be told that they don't have a car for you.

The reservation system probably overbooks to some extent because a certain percentage of reserved cars are never picked up.

Many rental outlets are franchise operations. It may say Enterprise or Budget on the sign but it's Joe's Car Rental LLC running the place. They may be slow at updating rentals and returns in the system. It's not all one homogenous company.