The hydrofarm is an on/off which means it turns on to reach the temperature set point and off until it reaches a lower temp and then back on again. Usually the differential is only 2-3 degrees so click on at 90 and off at 93.
A proportional thermostat keeps the same temp constant and the heating source is on all the time. The thermostat just limits the power going to it (like a rheostat) and adjusts the temp accordingly.