The way I understand it, franchise locations have two types of cars on their lot: "owned" cars that belong to them, and "pool" cars that don't. The "owned" cars stay at that location and the "pool" cars come in on one-way rentals, etc. The desk agents at franchise locations are under great pressure to rent the "owned" cars first for local rentals, in order to keep fleet utilization high and reserve the "pool" cars for one-way rentals. That is why you will sometimes see the car you want on the lot, but the agent will press you to accept a minivan or something instead. I learned this after I reserved an intermediate car with Dollar at MHT. The agent pressed me to accept a big, muddy, knobby-tired Jeep Wrangler even though there were three or four Dodge Stratuses just sitting there. She insisted the Wrangler was an "intermediate car" and was "similiar to" a Stratus. Fighting, sometimes at length, is the only option. I won, but I'm also permanently done with Dollar.

I use Avis almost exclusively now because when you're in the Wizard / Preferred thing you can reserve and pick up your car without interacting with any human being, and it's usually the car you ordered, within one class or so.

All car rental firms design their processes and fee systems to screw the customer, and it is definitely worse for neophytes and visitors from overseas who may find themselves paying hundreds of dollars in unnecessary fees. Dollar is among the worst.