Bear has a great post here about temperature swings. There are a few factors. If you have a Gen 1 smoker (I do), then the heating element and the top vent are both on the right, so the right side tends to heat up more than the left. Over time, the temperature will stabilize (it does this in your oven, too, but I know I never even checked my oven, so this was new to me as well), but it's very likely that your right side will remain hotter.
Also, here's an interesting thing. Say your smoker starts out around 60 degrees F and you set it to 230. The heating element will remain on until the sensor detects 230 degrees and then shut off, but the temperature will continue to climb up to some point - it could be another 15 degrees or another 50, hard to say. At that point, the temperatures in the smoker are more evenly distributed (not perfectly, but more so) and it'll start cooling. Once it cools to maybe 228 or so, the element will turn back on - but the temp will continue to drop. You get into a wild swing.
Bear talks more about this (sorry, I don't have the direct link) and has a great solution that works for me. Set your initial temperature lower than your desired temperature. Then as your smoker goes past the set temperature, you can then bump the set temperature to your desired temperature.
Another thing people do is put a small deflector on the bottom shelf (on the right side) to help the left and right sides be more even. Search for "defelector" on these forums.
BTW, if your smoker runs hotter than the MES readings, that's fine, just set it to a lower temperature and be happy you have extra head room. :-)