The emissivity is only one of the factors involved.
The temperature indication is also affected by the the angle between the line of sight and the target. You will see a maximum reading when the device is pointed straight on at the target. The indication will drop off if you angle off in any direction.
Another factor is the size of the field of view with respect to the target. As you move farther back, the field of view will increase. If the field of view grows to include objects that are at different temperatures than your target, then the temperature indication will go up or down according to the weighted average temperature of everything the device can "see".
In other words, to get consistent results, you should shoot straight on at your target from the same distance each time. For small targets, get as close in as the device will allow (look at the spec sheet to find what the minimum sensing distance is).
By the way, some of these IR temperature guns have a laser pointer built in which helps considerably with hitting your target consistently.
Best regards,
Andy Hall