Ok every tool has limitations. IR guns are clibrated to be pointed at something about the reflectance of a paper bag and be at close perpendicular not an oblique angle. The other issue is that even the most expensive can incorrect. They have a tolerance of 1-3ºF plus or minus. The best are one degree and the average are 2 degrees and some high range ones can be 3 degrees.

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Digital thermometers can be resistive or thermal couplers. resistive are cheap and couplers are not. Accurate digital thermometers have platinum tips at about 1600$ an oz they are never going to be cheap. Than means it is likely you as we all (mostly) have cheapo inaccurate digital types, that also have a typical 2º up or down error and still be acceptable by the manufacturer.

That means if your gun is high and your digital thermometer is low... They can be with in their specs but read 5º different. The solution is a single more accurate tool that can be used to check the other tools against. I recommend a cheap liquid Crystal type (stick ons measure the surface they are stuck to so that is what must be measured and glass will simply not work with ir guns. I use a one intended as a photography darkroom thermometer and it is accurate (tested against a science grade traceable accurate digital) to 0.5ºF and was $ 2.50 in fact I have 8 and they are all the exact same reading. Check in the store if ten thermometers are exactly the same they are likely quite good, if they are all over the place not so good.