Temp gun pointing directly at the glow plug will read higher temps than reading it just to the head. If you have an on board temp guage they have a loop that goes around the head and will also read somewhat lower temps.
Most people that I know put the temp gun directly in the center of the head pointing down at the glow plug and move it around some to try to find the highest temp.
Temop guns also vary quite a bit and can get dirty and not read correctly.

Rex