Power supply recommendation's


I have a question for the people here who help on builds, etc...

Why is it that people always recommend a 850w psu for the gtx 275's and / or the gtx 285's in SLI but yet they recommend a 750w psu for cards such as an ATI 4870's 1gig in Crossfire. I find this confusing when all the review sites show the gtx card using less power consumption than the ATI cards. So what gives ?