One thing I learned about the Accu-Rite thermometer/hydrometer is that it measures humidity through a small opening on the back of the unit. If you're using a particulate substrate like aspen or the like, it can block that area and you will not get a good humidity reading.

The stick on ones can be even worse, as Dr. Del said. I'd check where you have your Accu-Rite and make sure the hole isn't blocked. Then see if your reading is any different.