You seem to be confusing "available for purchase" with being available for you to get as an UG. There are many, many scenarios where there is no room available for your UG yet there may still be a room available for sale.

Scenario One: You are checking in and a Plat will be checking in later. The room is being held for the Plat, but if someone comes into the hotel willing the pay for the room, the Plat will lose the UG.

Scenario Two: The hotel has one room left available for the night and gets several walk-ins on an average night. The only room left is a more expensive room, but the hotel has a history of selling the more expensive room to walk-ins. Is the hotel supposed to forgo likely revenue just because you reserved a less expensive room but want more? If I were running the hotel, I wouldn't be looking at the more expensive room as available.

Scenario Three: You look online and see a room available, but when you go to actually reserve the room, you get an error message. (That has happened to me several times.)