I believe the hydrofarms are 3 degrees below the set point and one above. They also have a large accuracy range (+/-2ºC about 4ºF up or down). Part of the issue is not as obvious or as easy to detail. The probe is metal and slower to react to changes than most. This can add to the swing fairly dramatically, a heat source that is quick to heat and with a thin enclosure material the heat source heats faster than the probe responds so instead of shutting off just above the set point it continues to over shoot this as the probe starts to change the same on the cooling cycle the probe having more thermal mass than the similar sized part of enclosure it takes longer to cool thus dropping farther than the expected swing. Typically a cool room temp and a thin wall material like a tub accentuate the 'swing' producing higher and lower than even the 4º range. Add to the error potential of the stat this can be exceptional (set at 90 the stat will shut off at 91 and on at 87º this is correctly functioning and perfectly calibrated. I have never seen one (I own 7) that was even approaching correct temps. This adds to the issue, I would recommend a good accurate thermometer one of at least 1º +/- accuracy. sadly they can be quite costly thus reducing or eliminating any savings.
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