upon first starting the fan speed stays low until the coolant temp comes up (unless its really hot inside the car and you want it cooled down ofcourse) then it will ramp up to get the car up to temperature before ramping back down again.
Once fully warm if you adjust the speed it will USUALLY stay where you've set it, unless you've chosen a speed thats too slow to maintain the temp selected, when it will slowly nudge the speed up in order to maintain the preset temperature.