The thermos they put in the Weber lids aren't that accurate. They're bi-metal thermometers and even the best of them can be accurate one day and 50 + degrees in either direction off the next, and then accurate again the next. (Ok maybe not the super super super expensive crazy ones, but the ones they're sticking in grill lids anyway)
You have 2 things going on, 1 the fact that lid temps will always read a bit hotter than at the cooking grate temps, and 2 that the bi-metal thermometer in the kettle lid can easily be way off. Ignore the lid temp, and make sure you have your maverick placed in a good spot and you won't have to worry about anything.