the temp is only a guideline.
not definitive.
some pple use a $30 temp probe, some use rayteks, some use exergens. i would expect the temp reading to differ from one to the other
some point the temp gauge on the fin, some point on the case, some point on the glowplug.
the best is to tune ur engine by sight and sound. if u see a good trail of smoke and your engine does not sound lean, then why worry how much the temp is?
i only use the temp gauge when i want to measure RELATIVE temperature - ie if my engine is running at 110C and i tune it, then i can use the temp gauge to quantify how much a difference the tuning made, ie +5c or +10C, etc ..