I think the TDP is rated for the Whole chip to be loaded at 100%. And they did not take in to account what huge parts of the chip are disabled.

Look at the stats:
VS HD5850: 75 MHz higher clock 20 Watt more
VS HD5870: 50 MHz Lower clock 13 Watt less

It would not surprise me if the reviews show that the HD5830 consumes less in furmark and games compared to the HD5850.

And the ROBS who say's they can keep 24 active ? Maybe they only have 2 clusters of 16 ?