Humidistat Accuracy

[Farrell]

I have a big pile of copies of a full-color beautiful Damp-Chaser marketing handout that describes how the use of their dehumidifier system keeps the relative humidity (RH) of the atmosphere in the area of the piano soundboard at maximum of 42% RH. Why is it that when I place the probe of my NIST-certified 1% accuracy hygrometer on top of my Damp-Chaser humidistat, it reads 55% RH? When I place the probe on top of the soundboard it reads 55%. When I place the probe in the middle of the room where the piano resides, it reads 55%. The heater rods are not warm. The atmosphere next to the soundboard should be 42%. The room and  everywhere else around the piano is 55% RH. The humidistat should turn on at.......something near 42% RH.  I'm looking at the pretty graph:  http://www.dampp-chaser.com/main.html  It suggests that RH won't even come close to 55%. Where is the discrepancy?

Terry Farrell