generally, the temp is pretty even throughout the WSM cooking area. Placement of the thermometer will impact readings. For example, readings toward the edge of the grate will tend to be higher because the heat is coming up the sides around the water pan. Near the top of the lid, there is also air flow that affects the reading. A short lid thermometer stem is not best.
The best thing to do is find a consistent place to monitor temps and forget about the rest. If your cooks are taking a little to long even though your thermometer reads 250, then leave the thermometer where it is and let your temp come up to 275 or 300. Use your thermometer as a relative guide, relax and you'll enjoy cooking a lot more.