Cover your tank with foil and wait about 15 minutes for temperature equilibration – the number one source of temperature read differences is an uncovered tank due to evaporation and temperature instability.
Please keep in mind Anova immersion circulators are calibrated to a scientific platinum primary standard which is far more accurate than consumer-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.