Not surprising, really. I have my current corn snake in a 10gal with aspen bedding and a really shallow water dish. Nothing in there ever gets sprayed and the hides and branch are always dried before being replaced...and yet my humidity in there is almost constantly around 60%. He doesn't seem to mind one bit as he is quite shiny and healthy and his two sheds with me have been spot on perfect.

It is my idea that the small confinements of the tank and the depth of substrate are what have the humidity levels so high. Add warmth from the UTH and nearby lamps and there you have it. Sift and mix your substrate with your hands and make sure there are no wet spots underneath and just let the tank air out a bit, then replace the thermometer/hygrometer near but not on the substrate and see what you get. That be my suggestion.