There is a great deal of variation possible in all types most IR guns are +/- 2ºF or more most digital thermometers are also +/- 2ºF same for many t-stats the hydrofarms i have tested were more than 2º high.
Check the accuracy. Look up the specs of your thermometer and check the accuracy. They are all instruments and have limitations. It is likely both are with in the expected accuracy it is just that you are asking more from them that they can deliver.