... when someone doesn't like the room they have been assigned, for whatever reason, and they go to the front desk to ask for a different/better room, where is this supply of different/better rooms coming from?
It may be a room that was not booked or a room from a guest that made no requests. Yes more than 50% of Guests make no requests. They just want an available room as soon as they walk in the door. They want to enjoy the parks and pools and don't plan to spend all day in their room looking at the a relaxing and positive vibe of their feng shui resort room.
s the complainer being given a room that was previously assigned to and intended for someone else who hasn't yet arrived?
Yes but not from a guest that has made requests on their reservation. The Front desk CM often has to check with the magical back office to approve a room change. And as a reminder, requests are just that and 1 or more requests may not be honored. Easy requests are the most frequently matched. Upper floor, near the elevator(s), and near pool or transportation options.
The funniest requests are often concerning a sunny balcony in August when the "real" feel temperature is often above 103 degrees.
What happens if the complainer already entered the original room?
They are often stuck with the room for first night or 2 and then offered a move to a room that may better match their specific requests.
It bothers me to think that if I arrive late multiple families may have already been inside my room checking to see if it met their satisfaction and then I arrive and get the rejected room.
You don't need to worry about that as you rush to your Disney resort. But if you feel better just make a request for a room that has not been rejected by multiple previous guests. I am just joking at this point if it is not obvious by now...
Dave