My take on it is that looking at just the performance here is misleading. The 6850 & 6870 aren't designed to win a performance war, they are designed to win a price war and are very suited to that purpose. They have amazingly small chips, are pin-compatible with Cypress (little board R&D, re-tooling or stock write-off cost) while being more-or-less competetive with all of Nvidia's offerings in the segment.
AMD have much more scope for dropping prices that Nvidia, who currently appear to have brought a knife to a gunfight, and judging prices at launch seems a bit pre-emptive to me. IMO AMD will have wanted to avoid the the pricing problems they had with Cypress, it's better to start high and drop the price than the other way around.
Shhh! Sense has no place in the graphics sub forum!