That's clearly another consequence of "You can book any car with any rental company and cancel up until the minute of the rental or just not show up and there's no penalty.": they've run out of the smaller cars and only have one larger one left (e.g., more than X% of the people with smaller car reservations showed up, but less than X% of those with larger cars - and if you think this through, I think you need to bias towards larger cars in your planning to avoid not being able to provide an at-least-requested-size car when the opposite case arises).