I still think AMD is sticking to the plan of beating Nvidia with dual-GPU cards (plus they get to keep decent yields). It's not like they couldn't make an even bigger chip, I think they chose not to.
That way they don't get many bad chips, and both 6950 and 6970 don't cost a lot (and thanks to the high yields they can even offer such a luxury as lower part unlocking).
They might be in for a nasty surprise when Nvidia's dual-GPU card comes out, though...