Hydrofarms are not known for accuracy.

I have 7 now (work related and failsafes) and not one is correct, they all read high some by as much as 4ºF. Accurites are also not particularly accurate I have owned 3 and the best was 2ºF in error. The problem with cheap thermostats they are not accurate and same for cheap thermometers, accuracy is suspect. My advise is to trust neither. I always recommend buying 3-5 thermometers and testing them all against each other typically some will be reading the same and few few will be of of this point. I discard the ones that read away from the group. I use the cheapest thermometers I can find I usually only pay a dollar or two for them. ESU make a cheap one. most digital thermometers have an accuracy range, the most common is +/-2ºF I believe accurite is actually +/-2ºC (4ºF) making it greater than most. The hydrofarms tstats are also rated at 2ºC so it is very easy for one to be 4º high of correct and the other to be 4º low of correct and that is if they are acting as the manufacturer designed them.