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.
If you want your system to match your personal thermometer:
Use the calibration function to adjust the display offset of the read to match your thermometer.
This can be achieved by turning the device fully off. Start tapping the top-right corner of the Anova One screen when you turn the device back on and you will be taken to a calibration menu. From there you can choose the offset amounts.