The “field of view”, where every infrared sensor works, is cone-shaped: this means that the farer you go from the surface, the bigger the cone’s base – the area of which you take the reading – becomes.
If, for example, the distance from the center of the forehead is bigger than expected, the thermometer will include in the measurement area also the temperature of the eyebrows, of the eyes or of the hair. If the distance gets even bigger, it will include the temperature of the whole face.
In all these cases, the reading temperature would not be correct as it would be an average of all the parts of the face included in the “field of view”.