Everyone talks about getting a "good" thermometer, or a calibrated thermometer but I doubt most people are. Calibrated thermometers are fairly expensive and getting a cheaper one off Amazon is kinda hit.
I bought two bluetooth thermometers off Amazon and supposedly "calibrated out of the box" and of course they were both reading different temperatures.
I ended up getting a couple of mercury thermometers off eBay (can be hard to find due to rules). I put them in with the eggs and used them to properly calibrate the bluetooth thermometers. I'd recommend getting something like a mercury thermometer to properly calibrate your digital thermometer.