I've had more than a few cars that needed the rear of car lifted about 1/2" or so in order to calibrate SUM properly. Lowered cars can have calibration issues as well. iirc the level sensor voltage should be around 2.5v front and rear. I know its supposed to look at what the level is and calibrate from there but seems if the rear is out compared to front it doesn't like to set.
From what I've experienced it can make the headlights dip down towards the ground. Sometimes this is due to the headlights being adjusted up and then a SUM cal being done. Other times a faulty level sensor will also cause the headlights to dip down. I always try to get the sensors close to 2.5v before calibrating the SUM, afterwards should be ok if sensor read a little different, its just telling the SUM the level of the front or rear.