On one occasion, I've tried booking straight from the hotel, it told me no rooms available for the selected dates. I've tried Booking, and sure enough, that same room I had my eyes on was available.
Larger hotels have blocks of rooms reserved for the different booking services.
The opposite can also happen, Booking shows no rooms, and calling the hotel results in a room. It just means Booking's block of rooms is all rented out, not that the hotel itself is all rented out.
