Hmm.. Do you have the screen covered by anything?

If not, I think there is a problem with the hygrometer, unless your house is just super humid normally, which generally isn't the case.
Dito - most tank setups like this are too dry! especially with the heat lamps and lights above.

I'd suggest misting the glass and/bedding in a small area and see how long it takes to dry. If it drys in a few hours, I'd say there's a problem with the hygrometer or it's getting moisture on it somehow. What's your humidity outside the tank??