The dome it the kettle concentrates the hot air as it rises, you are getting a measurement of the air, which is heated, at the top of the lid. The air at the grate, for various reasons, is not going to read as hot. This has always been the case, and is the case for all smokers. I use a long probe thermo in the top vent, but, the tip is only 2" above the grate. Of course, if the kettle is fairly full, which I prefer, this thermometer cannot be used. I also use an over thermometer, and it has worked fine for me. I also have a Maverick, which also works, but, is more bother when I am doing something really simple, and all I want is a pit temperature.
In truth, I often cook without a thermometer at all, as I know my cookers and have a good feel for where they are sitting. Best to not have to rely on equipment.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."