Also, you may find that the room you want is actually available even if not marked as such on the aggregated list. Unless the smaller hotel is doing business exclusively with such website, they will have to spread their available rooms manually across multiple services and manually close them once they become reserved.

That means some room may be marked open on one website and another room on another. If there's a large number of rooms open, there may be some overlap... but then overlap means a possibility of overbooking. I've seen the systems that small hotels have to deal with and really I'm surprised we don't run into overbooking more frequently.