Ah, I see. I wrote underclock. It's really undervolting that gets you the big win thermally. Underclocking should be done just as a means of achieving a greater undervolt. I just have these two things in the same mental bin.

When you underclock properly (with SpeedStep) it also lowers the voltage... probably to the same voltage that the lower-bin processor would use.

There's not just one voltage here. It's a curve. I suspect that the better the processor turned out, the more favorable your curve turns out to be.