Post a pic of the whole enclosure, and try to include in the pic lighting and top.

Most humidity will escape through the top so the less screen material and more glass or plexi coverage you have the more humidity is less likely to escape. Also, lighting and excessive ventilation will have a big effect on humidity. Placement of the gauge is important too since you want to check the levels at the area the frogs are and not near the top.

One thing to remember too is that many of these gauges are not perfect. Since your firebelly tank is a lot of water it would be hard to fathom the humidity dropping to 10 percent.