Been discussed a few times. A few of us have tested ours using boiling water and they test out fine. I have theorized that the shorter stem (1.5 inch) on the Weber therm accounts for the different readings we new owners are getting compared to owners who have installed 3-5" stemmed Tel-Tru's in their older WSM's.
My grate temp using my Maverick ET-73 consistently reads more than my dome temp on the Weber therm and both therms test out fine in boiling water. Whereas users with Tel-Trus often report their dome temps being higher than their grate temps.
All of that said, if you have a full charcoal ring of lit and all of the vents open, you should be able to run the stock therm up well past 200, even given the different temps seen on the Weber therm...I'm able to do 200-225 with 10-20 lit briqs in a few minutes. I suspect I could hit 500+ with a full lit charcoal ring.
Are all of your vents open? Where are you? Altitude? What is the ambient temp? You should probably test your therm in boiling water to be sure.