Rental car availability at an airport location can vary greatly depending upon a large number of factors. Without access to proprietary information and date-specific local market conditions, it's very difficult to predict availability.
Example: Orlando has a massive convention center that hosts lots of high profile events. If there's a national convention that runs Thursday through Sunday and you arrive on a Friday, you have not have many cars from which to choose. That lack of vehicles might be the same whether you arrive at 11am or 11pm.