Hmm, that 130 w more, that's the total system consumptions comparison, right ? not just the comparison beetween the power consumption of the cards themselves. The system with Fermi in it might work a bit harder so not all of the power difference goes to the card itself. And it still has to account the efficiency of the PSU.

But i think, and it seems, ATi is more conservative with its TDPs on HD 58xx series compared to nVidia Fermi's TDPs.