When your cooker temperature doesn't match your external thermometer source, it's due to heavy evaporation and inadequate waiting periods. Cover your tank with foil and wait for about 15 minutes for temperature equilibration. Once the water temperature is stable, complete a second read.
Please keep in mind your cooker is calibrated to a scientific platinum primary standard which is far more accurate than consumer-grade thermometers. Candy thermometers and meat sensors are not accurate enough to warrant a calibration. We recommend a Thermapen or higher grade thermometers.