IR Thermometer is non-contact style measuring tool with certain distance to get surface temperature by receiving all items' infrared. Basically this may let you know the accuracy would be larger then contact style, especially while they are still some distance existing between measured target and thermometer.

Why 2 degree C or 2% of reading whichever is greatter? Because of above issues, the world leader manufacture of Infrared Thermometer, Raytek (now is owned by Fluke), found all item have different emssivity which impact temperature reading. But lucky,  most of item would be near 0,95 area.  Most item in the world is between 0.93 to 0.97 which means there would be may some accuracy issues involved. Thus they follow the optics and  statistics theories creating 2 degC accuracy.

Since Raytek is the leader in this industrial, every body follow their rules as guideline. Of course, when you check all IR thermometer in the market, you will find this accuracy is almost a principle  for whole industrial.