The answer is, in a word, resolution. Take a look at the image below.
The infrared camera is able to locate and measure much smaller objects than an IR temperature gun. Note the spot measurement on the infrared image; its reading is 250 F. Contrast that to the average area measurement typical of an infrared gun at a reading of 184 F. The gun is averaging all the hot areas along with the cold areas of the grating we see in the image.
Another common question is why the IR camera reads a higher temperature than a contact temperature probe? Contact temperature probes require an extremely good contact in order to read a decent temperature. In fact these probes measure their own temperature, so if the heat transfer is not good from the substrate, and the probe sensor is colder than the surface, the temperature will be lower. Also remember that contact thermometers act as heat sinks, sucking heat out of a surface.