I am also in the 'hate the first floor club' -- I suspect HIS/HGI are more common offenders because full-service properties frequently dedicate the entire first floor to "public" space (restaurants, meeting rooms/conference center, atrium, etc.) so a room on the first floor is simply more likely to exist at a HIS/HGI as compared to say a Hilton, WA, or Embassy Suites

Once, at one of my most common properties, a HGI, I OLCId into my 3rd-most-favorite room (top floor, far end of the corridor, and for reasons I've never figured out it is significantly larger than the average room if its type. While I was in transit to the hotel I got the "we're giving you a new room -- and it's ready!" notification only to find that I had been moved to not only a room on the first floor but the one closest to the front desk, elevators, and fitness center. Called the hotel almost immediately with a "hey, I understand you had to change the room I selected but I really hate the first floor -- is there any other room available?" the FDA was very understanding and put me in the shotgun suite right next to the room that I had originally selected (and also threw in an "I'm not sure why someone would have moved you there" -- not sure if that came from seeing my "High Floor/Away from Elevator" room type preference or just common sense that if someone chose a room on a high floor...)

I think it's something along those lines but I don't think it's directly due to housekeeping updating to vacant/ready (otherwise when a hotel is sold out the night before you'd never be able to select a room and/or I'd never a get a "oh, sorry, housekeeping isn't quite done with the room you selected, would you mind a different room or prefer to wait?" at check-in. I suspect it's a combination of canceled reservations/early departures plus a dash of released room blocks and maybe a hint of housekeeping. I remember a couple years ago in Switzerland the first time I looked there was one, undesirable, room available. Later there were a couple dozen rooms but none of the single-bed variety available (and this basically stayed the same but the specific rooms changed), and finally at check in I was assigned a room that never showed up in any of my OLCI attempts.