Temperature controls like this aren't exact. Besides not being calibrated well, there is a lag between the cold air circulating and the temperature of the thermometer itself. And like a few people pointed out, the items you put in the fridge retain their temperature better than air. For example, if your fridge is empty and you open the door for five minutes, you let all the air out and it will get warm, whereas if you have items in there that retain the temperature, it would remain cooler. Apply that to a thermometer now. If it's just sitting in the air it will tend to be warmer than if it's next to a keg or in water or something that would maintain a more constant temperature.