There isn't a finite pool that the various channels are drawing from. It's not like flights where there are X seats on a particular flight and they may overbook by Y% based on a perceive no-show factor or Z%. With car rentals, it's a bit more fluid:

- You have one-way rentals with vehicles being added to and subtracted from a particular locations fleet on a daily basis
- Since most rentals are pay-later, the no-show rate in the car rental industry can be as high as 30% in some cases
- Renters often return early/late
- People are often given a different vehicle class than booked, either due to paid or comp upgrade, self-selection, lack of availability, etc.

The bottom line is that when you rent a vehicle, regardless of channel, it's not like they're taking a specific vehicle and allocating it to you. It's all a big fungible pool and they manage capacity based on the utilization level of a particular location and vehicle types on a day-by-day basis.

Specific channels don't typically have"enhanced availability" and there really isn't such a thing as a "non-existent rental". if the rental company is willing to take a reservation they are obligated to provide you a vehicle. That doesn't mean they don't sometimes overbook and fail to deliver, but for the most part, they will work to accommodate you one way or another.