From the FSM: "Evaporator temperature is controlled by a fixed setting thermostat switch which cycles the compressor clutch to maintain minimum evaporator temperature."
Since the temperature of the evaporator is affected by the amount of air blowing through it, and the ambient temperature of the air in the cab, it acts like a climate control passively. EG, if you set the temperature to medium and turn the fan on low, you won't blow a lot of air through the evap and it won't shed cold air and the thermostat will reach the cutoff point sooner than if you had it on max cold at max speed.