They downclocked the card to hit a power consumption target. They have a stated tdp lower than 6970, but it consumes more than the 6970 in games.
They also probably wanted to hit a power consumption target less than the gtx 580 because of the increasing power needed to get more than 1ghz for the 7970. And lets be real. Conservation/power consumption/being green is hot now. Even at the high-end, power consumption is a bigger issue for everyone (including enthusiasts) in the past year or 2. How many people purchased overkill psu's 3 years ago versus today?
Saying the card beats the gtx 580 by 20-30% while consuming less power is better than saying it beats the gtx 580 by 30-40% while consuming more power.