Don't know if they ever have, but now they are "Auto-Calibrating" or so they say. This is why every one I own has a little white sticker on it saying +4 or -2 after I check the calibration.
I bought some at Wal-Mart and every one of them was spot on. Put them in a sealed bag with a 75% Boveda and 12 hours later each and everyone read 75%. Downside they are a little big for a desktop but great for cabinets. Around $9.
If they were all calibrated, and stayed that way....they would not have a need to sell calibration kits! Sorry, but it's the truth. It's pretty old and simple technology to not have perfected by now. Do the outdoor, wireless kits need calibrated? Nope, and they measure a lot more than rH and temp. Chew on that for just a minute.
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My favorite cigar list here