there are alot of factors going on in a UDS that make the temperature question hard to answer
keeping in mind that thermal energy can be transported via conduction, convection, radiation or by a combination thereof, along with effect of latent heat. the only factor that can be left out is pressure, since effectively the pressure is constant, with intake and exhaust in a fixed state ( air in = air out).
latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase (i.e. solid, liquid, or gas), – also called a phase transition.
With some moisture being driven out of the cooked food and fat rendering they both absorb the heat. fat or water dripping onto the coals once again absorb heat as they are vaporized. drippings and fat can also extinguish areas in the coals which can diminish the available heat for a period of time. fat can become fuel and enrich the available heat
your temp gage only reads the "dry bulb" heat in the chamber, while all the other forms of heat energy are left unmonitored. The moist air in the chamber is more like steam than the hot air your temp gage reads.
Radiant heat travels from the heat source to the target with out heating the air in between , your gage may register some of this radiant heat due to it's stem being a small "target" . the 'heat shadow' effect that BBQchef33 mentioned can be thought of like an eclipse. Radiant heat travels like a beam of light, so anything that is hidden from the beam will NOT receive radiant heat.
convection heat can be described like holding your hand in front of the heater vent in your car, blowing heated air transfers more heat to items they are passing by, your hand gets warmer faster than the car. Again your gage may not register this heat.
Conduction heat, well in its simplest form ... think frying pan. the food in the pan will absorb the heat directly conducted from the pan. if there is a source of heat below the pan the food will eventually reach the temp of the pan. in the UDS the conductor is the smoky moist air. this is what your gage reads.
Sooooooooo 225 degrees is NOT 225 degrees. its only the average temp of the conducted heat in the air plus a small amount of the radiant heat. the rest of heat energy is totally ignored by the temp gage.
and yes the convection heat source ( the air) is 225 but it conducts more heat energy. The Radiant heat may be more or less than 225 - but odds are good that it is higher.
Practical experiment - take a non radiant heater thats has a fan and directed it to a chair, then take a radiant heater and aim it at the same chair. try the heaters on with the fan and radiant heat going - then reconfigure to no radiant no fan - you will get the idea in a hurry