Update on this situation:

First of all, yes I did purchase them from Rich at Reptile Basics. He has been very good, exchanging quite a few emails with me and the problem is resolved to my satisfaction.

Here is a quote from one of his emails (posted with his permission) that might be useful for anyone else that runs in to this problem.

I did some experimenting with a couple of units here and also heard back from Johnson today. They confirmed what my suspicions were. Turns out that it is a mathematics issue with the sensor, not a defect. It boils down to the thermostat thinks in Celsius and it rounds the answer to the nearest whole number in some cases it can not get 83 for an answer. It switches on and off in Celsius even when it is in Fahrenheight mode, it just displays fahrenheight in F mode. Seems that is knows 83 is there but just can't say it. I noticed that when I held the probe on one here it would climb up steadily and sort of pause between 82 and 84 and then keep going. Johnson guy said they most often hear about the stat missing 47 degrees. Apparently that is the other possible black hole. Long story short, it is technically a defect though not really. Seems the unit works fine just doesn't display just fine on some in F mode.