Your smoker bowl only has so much surface area to absorb energy from the charcoal. Also the charcoal temperatures are >212F and there is not enough surface area/contact time for the bowl temperature to equalize with the flame temp. The warm gas then mixes with the steam created from the water pan (boiling at ~212F) to get the final smoker temp. Usually there is also quite a bit of heat loss from the shell of the smoker you have to take into consideration as well.