Re: Thermostat/thermometer different temps?
When the actual (thermometer measured) temperature is higher than your thermostat's set point it means that the probe is likely not in contact with the warmest part of your heating device. The thermostat is probably starting to read the cooler air temperatures around the heating device and trying to make up for it by sending additional power to your heater. My first check would be thermostat position and/or security (make sure it's where it's supposed to be and that it doesn't move). If that all seems good, as KMG eluded to above, it could also be that your substrate is acting as an insulator. You don't want to have too much down otherwise it will start to trap heat underneath it.