Yeah, I had to question you about checking your lab thermometer because it seems to me they sometimes get a bad rap. But I've owned a couple and they were dead nuts , so I have to wonder if people are sometimes checking them wrong.
BTW, not sure what you meant about calibrating the lab thermometer. When brewers say "lab thermometer" they mean the liquid filled glass kind that you can't calibrate. And you don't really ever need to adjust a thermometer anyway-- the important thing is to know the error so you can add/subtract for your readings. E.g. I have a digital that I can't adjust, but I know it reads 3f high at mash temps. Good enough.