One word...... emissivity. Temp guns may read consistently, but not always accurately to a given degree.
Most all the little "cheapie" $40- $50 temp guns have a fixed emissivity which leads to errors on varying colored/textured surfaces. Black has an emissivity of 1. White, silver and translucent surfaces have a much lower emissivity and this is why many temp guns have a fixed emissivity lower than 1 (like 0.95) which leads to accuracy errors of varying degrees.
Energen makes the only certified NIST traceable temp gun in the industry at a price point of ~$300. They have a microprocessor control that automatically compensates the emissivity. Having owned several $800- $1000 full blown R/C nitro burning engines, Exergen is the only temp guns racers can truly rely on the manage their engine's temperature and performance. Overtemp a nitro engine by 10% and you will have a pile of parts when the glow plug drops at 45,000rpm on the back straight. At 300F you do not have much leeway.
The best tool we could probably use is a contact pyrometer which is not practical for egg boxes and are quite expensive as well. The next best thing is what most all of us use.... Herp related temp probes. They have no emissivity errors since they are contact probes and are not prone to the same errors as temp guns. Far more reliable IMHO.
Just a spec sheet from an Exergen DX501:
http://www.exergen.com//industrial/dxseries/broch.html
Here are a few quick videos on the emissivity error of a $40 temp gun. This is probably the maximum error using a reflective surface vs. black electrical tape. Would you like to trust your eggs to this?
Test #1 on warm water. What is most amazing the the degree of emissivity error closest to where we incubate our eggs. A 17F difference (~17%)!!:
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