because there is no such thing as a perfect ballast,all ballasts suffer losses,which means the ballast has to run at a higher wattage than the wattage of the lamp,for example while the regent ballast may be 175 watt,due to ballast losses it actually runs the lamp at around 150 watts,while a quality ballast is in the vincinty of 180-190 watts and the ballast losses means the lamp runs at the specified 175 watts
