Could you possibly contact the manufacturer and ask them why? And tell us what they say.
Personally that makes no sense to me as water depth has nothing to do with humidity as long as surface area is the same, just how long the water lasts before it runs dry. I agree with Percheron, surface area and temperature control how much water evaporates. But maybe since it somehow regulates the humidity automatically it has something to do with the instrumentation.
If the goal is to maintain a constant level in there, adding a certain amount on a strict schedule may not be what you need to do. If evaporation is not what they expect, and that can happen, the level may go up or down depending in how much actually evaporates.
I would be very reluctant to go against the directions without understanding why that comment is there.