Matrix 10 Ultra not using hardwood solution

I've had my Matrix 10 Ultra for about a month now. Most of my floors are hardwood, and are showing as such in the map, but after all this time the hardwood solution reservoir is still topped up. Maybe it's gone down 1/2 cm. Shouldn't it be using more?
Sorry I had to select X50 Ultra as the model because Matrix 10 Ultra isn't listed yet.