You should normally be getting reproducible results, or results within a few tenths of a degree of each other.
However, a few things to know:
Multiple temperatures taken over the same area in rapid succession will ultimately cool the area causing differences.
If sweating, the temperature on the neck behind the ear lobe becomes the overriding temperature, and since this area is not quite as stable as the temporal artery, it can easily cooled be by the thermometer in taking multiple readings. Remember, the probe of thermometer will be at room temperature, and is about 30 degrees Fahrenheit (17 degrees Celsius) cooler than body temperature.
Measure only the side of the head that is exposed. Any covering, hair, hat, etc. will prevent the heat from dissipating, and would cause the reading to be falsely high. This also applies if one side of the head is into a pillow.
Slide the thermometer in a reasonably straight line across the forehead, midway between the eyebrows and the upper hairline, and not down the side of the face. Midway on the forehead area, the temporal artery is less than 2 millimeters below the skin surface, but down the side of the face, it is much deeper and less accurately measured.
Source : http://www.exergen.com/medical/TAT/tatfaq.html