It's probably built as a general purpose/gaming machine, not a trimmed down mining machine. 6950s are 200W stock and far more overclocked for gaming, since the RAM is not underclocked in that case. That puts you at 800. Add another 350 for the base system and peripherals and you're at 1150. That's over the 90% mark for a 1200W PSU. Add a meager 25% for overclocking and expansion and you're already over 1400W. If it was just for mining, he could have gotten away with a 1200W, for sure.