People seem to really get a kick out of reading how this really works behind the scenes.
Here is an example to show you why you don't see your room assignment ahead of time, as well as how upgrades happen.
We have both booked standard rooms at a deluxe resort. You do online check-in and I opt to do regular check-in. You say you are arriving at 10:00 am; they don't know what time I am arriving.
About three to five days before, a room assigner assigns you rm. no. 123, and I am assigned rm. no. 124. This deluxe resort is not overbooked, so they hold rm. no. 125 in reserve and there is also an available concierge level room no. 625.
At 9:00 am, Mousekeeping calls and states rm. no. 123 has a broken faucet. Maintenance has looked at it, they are a bit backed up, but can fix it by noon.
Several scenarios can happen:
Scenario no. 1 - You check in at 10:00 am, and they give you rm. no. 123, but tell you it is not available until noon. I check-in at 10:30 am and get room no. 124, which is available at 11:30 am.
Scenario no. 2 - you get my room no. 124, which is available sooner, and hope I check in later than noon. I check in at 10:30 am, so they decide to give me rm. no. 125. Rm. no. 123 will go in reserve until the repairs are completed.
Scenario no. 3 - They give you rm. no. 125, which is available, and then give me rm. no. 124, which is not available until 11:30 am.
Scenario no. 4 - You ask if there is a free upgrade. The CM says there is an upgrade to concierge level and it costs "X" amount of dollars. You decide it is worth the upgrade and you take rm. no. 625. I arrive later and get my room no. 124. Rm. no. 123 will go back into inventory when the repairs are completed.
Scenario no. 5 - You opt not to pay for the upgrade and take the available rm. no. 125. At 10:15 am, the Front Desk receives a call looking for an available standard room, since a value resort is overbooked. This would be an upgrade for them. They offer them room. no. 124, which will be available to the displaced guests at 11:30, which is about how long it will take to get their baggage over to the resort.
I check in fifteen minutes later. My room has been given to another guest, and the other standard room is still having repairs completed. SOOOOO I am told I have been given a free upgrade to room. no. 625, and rm. no. 123 goes back into inventory when the repair is done.