They did over estimate a bit, at full load it would run about 450w for a pair. But I think they wanted to add a saftey margin. I personally wouldnt want to push it. Some people buy the cheapest psu they can find. I could see some kid trying to run a pair of GTX 560s with a 650w rocketfish or dynamax psu.
Also, 50-100W is a safety margin. 300W is stupidity.