I'm pretty sure it's the same chip manufactured on 55nm with less redundancy (and thus less transistors, making it a little smaller even) and a little higher core and shader clock. The other option is that the chip is the same entirely build on 55nm (maybe with some extra transistors for redundancy) with more active components. The first option is more likely as Nvidia really wants to make this chip smaller. If the second option is chosen it is only to not lose face, but Nvidia will lose some money over this.