It’ is normal for different brands of thermometers to have some temperature variance within a few degrees.
Try your non-Guru food thermometer in both boiling water and ice water to see how accurately it reads. It might be that your non-Guru food thermometer is out of calibration.
If the food temperature on the BBQ Guru control is about 15-20F hotter than your other food thermometer, make sure you have the food probe fully inserted into the thickest part of the food. If too much of the steel shaft is exposed, it will cause the temperature to read hotter since the probe is picking up heat from inside of the cooker and not the actual temperature of the food.
If your food probe is incorrectly displaying a very high temperature usually in the high 400F range, your food probe may be bad or failing. Buy a new probe here or see our warranty/return information.