The differential is the difference between the turn on point and the turn off point. It s usually about 4 degrees. This means that if the thermostat is set at 80, the blower will turn on at 80 degrees and turn off at 76 degrees. This gives the blower some working time. If the thermostat had no differential the blower would turn off as soon as it turns on, then off, then on, etc., not very efficient.
Some people turn the blowers on with a timer.
The blower might be set to turn on at a specific time or multiple times during the day. This will also get the job done, but it s not the most efficient method. If you re looking to control the temperature you should measure the temperature, not the time of day. Sometimes the air coming in is just too hot and can t cool the room. When this is the case the blower can be run on the same timer as the grow light, or another timer set to go on at the same time.