Airsense 10 humidifier tank never goes below 1/3 of a tank.

Can anyone explain what might be causing this issue?

It appears that regardless of the number of hours I utilize my AirSense 10, the humidifier water level never drops below 1/3. I have tried it for 5 and 9 hours with various temperature settings, but there is always 1/3 of water left in the tank. Perhaps I could experiment by not filling it up the night before and observe what happens to the remaining 1/3 tank of water.

Is your ambient humidity constant or does it vary?
Does your pressure ever vary?
Do you ever have big leaks?
All of the above will affect amount of water consumed.

The most common cause of variations in water consumption is going to be ambient humidity.
When it rains and my ambient humidity is rather high I use very little water but when the air is really dry I use a lot of water.

Well that's just the thing, it uses the same amount of water no matter what, seems kind of odd to me?

All I was doing was pointing out that there are several factors that can affect water consumption that can and will vary from night to night.

You might just have a really awesome HVAC system in your home.
Our ancient abode is not so blessed, so my water usage varies by season.