It sounds like there are three things the first being ultratherms are self regulated to not get over 100º in an 80º room ambient temp. They in cooler rooms will not even reach this.

The hydrofarms are very very poor accuracy they are supposed to be with in 2ºC of correct (about 4ºF up or down from correct) long story but I have a number that I have been paid to evaluate. I think there is 3 in the first batch and 6 in the second. 3 of them are beyond the 4º mark from correct. They all read high ranging from 1.3ºF to 5.9ºF from correct. There is likely an error with the T-stat.

Digital thermometers come it two flavours cheap and accurate. Most cheap (sub 100$) have an accuracy of +/-2ºF for correct and with age this can decline more.

It is possible your thermometer is faulty or that is is incorrect one direction say 2º low and the hydrofarm is 4º high that would mean the hydrofarm set to 90 is actually 86º if the thermometer is 2º low 86º would read as 84º that is if everything is in the specs! It is easy for both to be outside specs too.

It sounds like both are an issue to me Personally I would suggest if you choose a hydrofarm a correct reliable thermometer (Traceable with less than 1ºF +/- error) is a must. Sadly the price of such a tool is more than the savings on the thermostat compared to a better reptile specific t-stat.