^ +1 on what the others say.... Unless you are trying to reach a specific level of being done on a large cut (Rare, Medium) the thermometer is a guide, much like a mile marker on a highway.
I think most people only use the thermometer as a guide, and not as being an exact requirement to maintain.
Variables in fuel, meat, weather, and the cooker will all change things every time we cook. Up and down temperature spikes always occur, even with the use of a pit controller.