From what I've read, Tel-Tru is the gold standard for bimetal thermometers, an inherently "problematic" design that often suffers from inaccuracy, especially as it ages and/or at higher temps. Like a few others here, I have a BQ100 set into a Weber bezel on one of my kettles. Tested in boiling water at exactly 212, but don't know about higher temps, where it could be off.
Of course, any lid thermometer is an approximation of what's happening at the
grate. Also, there will be times when the lid temp is basically the SAME as the grill grate, because after a certain point (meaning, at highest temperatures from what I can tell) the temp sorta "pressurizes" inside/can't dissipate fast enough for there to be a difference. But at "typical" 250-350 degree cooks, yes I'd expect there to be a difference of 50-20 degrees between the two locations?
For most cooks that don't require longer investments of time I prefer a simple oven thermometer sitting on the grate, as can usually be seen in a typical 1buckie image ... I need a new one, the one I've been using for 2 years is now so dark inside it's nearly unreadable.
Mine (the oven thermo) is a cheap Taylor TruTempfrom Target,
http://www.target.com/p/taylor-trutemp-oven-and-grill-thermometer/-/A-11010614 and yes it's pretty darn accurate. Just can't read it any more. Smoke gets inside and that's it.
I'm considering this one because it can hang, stand or clip (which should allow viewing straight up instead of bending down?)
http://www.chefscatalog.com/product/21424-taylor-oven-thermometer.aspxI'm also very interested in this OXO ... angled, "snap-in" design might be perfect ...
http://www.bedbathandbeyond.com/store/product/oxo-good-grips-reg-oven-thermometer/1012859430... and I treat the lid thermometer as an indicator of rising/falling temps in case I need to adjust dampers or add fuel.
I realize most ... or all... of that isn't what you asked about!