A couple of things to look at. Where did you have the probe for the fireboard located? Near the built in box probe or down near the element? As mentioned by barelfly with nothing in the smoker to absorb the heat - intercept the heat between element and probe, you could get a high reading.
Your controller is of the capillary type. The following information is copied from the included link. It explains how this works:
"In Capillary thermostats temperature sensor consists of probe, capillary tube, diaphragms and expansion medium. When the sensor is heated, the liquid medium heats up and expands. The expansion of the medium increases in the closed-circuit system.
The pressure increase is converted into displacement in the diaphragm. This displacement, also called travel, actuates a snap-action switch, which opens or closes the contacts in the electric circuit." (
http://www.thermowatt.com/en/category/capillary-thermostat-electric-ovens#application%5B%5D=158&application%5B%5D=163)
Do you have an instant read infrared thermometer? This will not "prove" anything but take a reading of the element itself after it has been on 15 or 20 minutes. You will see that it is far hotter than what you set the temp for. This is normal.
If you have a modern HVAC system in your house that is serviced regularly in spring and fall, you might notice the technician taking readings at the registers. Ask him/her what the reading are versus what the thermostat is set for. You will find the air coming out will be far colder or hotter than what the set temp is. That is the only way to comfortably condition the air for the entire room/home.
If you are concerned, for your first cook, I suggest pork ribs. They are usually done between 4 and 5 hours at 225 degrees. I would just prep them and some them at 225 and check at the 3 1/2 or 4 hour mark for doneness.How much pull back of the meat from the bone? Can you slide a tooth pick into the meat at the bone? You will have to do your own judging. With your fireboard and proper probe placement relative to the built in probe, you should see not more than a 20 -30 degree temp swing high and low of the set temp as the unit cycles on and off. That would be typical. The final result is that the temp pretty much averages out to your set temp. And the food is good.