I would trust your Herpstat over a zoomed thermometer/thermostat. I had the same issue with my Herpstat not matching my Accurites. I ended up using my temp gun to try and confirm, and the Herpstat was spot on. One Accurite was under and the other was over. If you use a temp gun, make sure you're not measuring the temp by pointing it at eggs as it will read high. My eggs were about 4 degrees hotter than the ambient temp inside the egg box. I ended up just pointing the temp gun at the substrate and the temp probe to get an accurate temperature...