That's the standard excuse, and one that's impossible for customers to check. My favorite use of the excuse was when two desk shifts at the Ledra Marriott in Athens had "sold" their executive rooms until I made it clear that I wasn't going to yield on my guaranteed room type and wanted the elite benefit guarantee money. (The only rooms of my guaranteed type were on the executive floor, though I didn't know that until one miraculously became available. I guess "sold" is relative.)