A lot of hotels will also spread their stock over a lot of sites to limit overbooking and micro management. They'll put 5 rooms on booking.com, 5 on expedia, and 5 spread around a dozen other places. There's no confirmation step like with deliveroo because the industry evolved with much less real time communication. So as far as booking.com knows there is only a few left. At least that was the case >10 years ago when I worked on automating this stuff.