The relative humidity measured by the thermostat is highly related to the surrounding conditions. The thermostat's location might be causing a deviation in the room temperature measurement and this in turn leads to a deviation in the humidity measurement.

Check the thermostat's room temperature measurement, using a properly calibrated thermometer to do so. If the measurement is incorrect, see the following link ( The thermostat is at a distance from the room's habitable zone and the temperature difference between both zones generates discomfort ).

If the humidity measurement continues to be wrong, check whether your thermostat is properly installed. See: ( The thermostat does not measure the humidity well ).