I find in mine that the maverick thermometer at the center of my grate with a terra cotta diffuser will read cooler than the edge of the drum at the beginning of the cook, then it will read hotter after the terra cotta has heated up, and then much later in the cooker as more charcoal has lit at the edges of the basket it reads about the same.
I find that when I add meat to the smoker the temp on the thermometer will drop because I just added 20# of cold meat. BUT, the fire hasn't changed temp. It is still the same temp at the coals. If I open the smoker up to bring the temp back up, then I've just made a bigger fire and as the temp of meat rises, the smoker will creep up in temp.
I typically cook overnight. I find that it gets cooler outside and my temps can drop slightly. But, if I try and compensate, it is usually such a small change that changes in the ball valve setting results in overshooting.
Once it is locked in on temp, prove it to yourself by letting it burn at that temp for 1 hour at that temp. If it doesn't creep up, it likely won't throughout the cook. Then put the meat on and fight the temptation to adjust the settings. Trust that the smoker will revert to the temp you had before. There can be 20* to 50* difference in the temp between the edge and center. It changes throughout the cook. You must learn to take an average and be willing to accept cooking in a 20* temperature range, and not at a specific temp.
It takes practice. Good luck.
Proudly Smoking on a UDS, PBC &
Accidental Winner of the 2012 "Saucy" Throwdown.