Ahh yes. I have experienced much the same.
In my Bubba Keg if I ran a couple of my wire probed thermos with the wire between the lid and the grill it would quickly go to HHH or whatever it was that was too hot to read even though it was no where near the "real" thermal max. I deduced rightly or wrongly that the metal wire was conducting the heat to the probe via being in direct contact with the metal.
I think you are seeing some of the same results. Also, remember that the metal oven style thermometer will also likely conduct the heat right off the grate rather than the actual air temperature which can be two very different temps.
I think some of the higher end wired thermometers do a better job and or have better insulation to handle this.