The reason being (and these aren't my words)
"Do you have a light meter? 1000w lamps are too much for plants and causes photo-inhibition, stopping photosynthesis. At around 5500 footcandles photosynthesis stops, in nature. You can push up to 7000 indoors, but that's max, and still causing some plant shut down. If you back the light up to 2', then you lose 3/4 of the lamps energy, a 1000w light will only put out 250w of usable energy at 2 feet, its called inverse square law - its just the physics of light.
I recommend getting away from 1000w and switching to 600w for the sake of the plants. More isn't better when it comes to food or light, or CO2 or much else."
dude answers tech support questions at sunpulse . he pushes lamps and the 1kw SHOULD be more expensive with more profit for the manufacturer (so why is he pushing 600s if there isn't something to it?) . although at hortilux you spend more for a 600 than you do a 1kw super hps.