2 rooms/seats left etc is not true in most of the cases. The way it works that an OTA (online travel agency) gets a maximum of 5 seats assigned before they get new ones. Technically it's true from their point of view that there might be only 3 seats left before they get new ones assigned, however it is not true from the consumer's perspective who might be tricked into believing that there's only 3 seats available on the entire plane. This is also the reason the threshold number for displaying these messages is never more than that as they don't even hold that much inventory at any given time.