What kind of temp gun are you using? Most guns have a sensitivity of 1%. Some will even be worse than that, like 2% and higher. Guns also have a ratio on them of like 2:1, 20:1 etc etc. Which means at 20 feet away, your measuring a circle approximately 1 foot in diameter.
The lower the sensitivity, the better. So if something is "scientifically" 100 degrees, and it has a 1% sensitivity, your gun could say 99, 100, 101.

Or if it's "scientifically" 500 degrees, your gun could say 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, or 505. Make sense?

Hope this helps.

BTW I'd trust my temp gun before I trusted an acurite. But that's because I know I have a good temp gun with good batteries.