Hey there, Science, they didn't measure CPU + northbridge. They measured system consumption. Petz already explained there are other factors-- inefficiencies in power supply, TURBOJET fan requirements on Prescott, etc.
But I suppose they could have measured 5kW, and you'd still try to justify the results.
But they didn't, and you can't deal with the results they did measure, which agree with previous experiments of this nature.