When completely filled mine will run on high 24/7 for basically 2 days... But, yes... I guess you're right about it not actually holding 9 gallons of water. I believe the reason why it is referred to as a "recirculating" humidifier must be for exactly the reason you said. It must suck moisture back out of the air then pump it back out. So, basically, even though it only holds 4.5 gallons, it pumps out the equivalent of 9 gallons per day?
One REALLY bizarre thing about this humidifier that completely boggles my mind is, when you fill up the water jug completely it is clearly completely full to the rim. But, when you turn it back over to put it back in the unit it will only be 3/4 full??? Can someone PLEASE explain how that is possible??? I mean... I am a magician and I can't even figure this out! LOL!