Probably, the thermometer is not always aimed on the same spot and at the correct distance from the center of the forehead, or it  has been moved before the lights flashed: make sure you keep the thermometer perpendicular to the center of the forehead and at the correct distance indicated by the aiming system, and keep it steady while the lights flash. Changing the reading point on the forehead will give incorrect results. – The thermometer was stored in another room (or even in a drawer, cabinet, etc.) at a different temperature than the room where the thermometer is used. Make the MQCS or wait until the countdown has run its course (AQCS) or for 10/30 minutes before taking any other measurements, so that the device can stabilize on its own.