They probably missed the clock-rate by a factor of 2x, or decided to use half the number of chips and overclock them to save money. It may also have been their backup plan because people are willing to pay 8x as much for power, but they're less willing to get 1/2 the hashrate.
If you run a CPU at the same voltage, but at half the speed, it uses 1/2 the power because CMOS circuits only really use power as the transistors change state. However you're also able to run at about 1/2 the voltage without getting errors. Because V=IR, 1/2 the voltage at the same resistance is 1/2 the amperage, and since P=IV, that's 1/4 the power for normal circuits. Of course CMOS circuits already run at 1/2 the power. In the real world there's also leakage and such, but in a perfect world doubling the clockrate takes 8 times the power.
Transmeta's Crusoe was the first processor to really take advantage of this, by varying the voltage and running slower to save battery life. Before this time (2000), laptops would use a duty cycle to save power. Soon all the major chip manufacturers started copying them.
If you run a CPU at the same voltage, but at half the speed, it uses 1/2 the power because CMOS circuits only really use power as the transistors change state. However you're also able to run at about 1/2 the voltage without getting errors. Because V=IR, 1/2 the voltage at the same resistance is 1/2 the amperage, and since P=IV, that's 1/4 the power for normal circuits. Of course CMOS circuits already run at 1/2 the power. In the real world there's also leakage and such, but in a perfect world doubling the clockrate takes 8 times the power.
Transmeta's Crusoe was the first processor to really take advantage of this, by varying the voltage and running slower to save battery life. Before this time (2000), laptops would use a duty cycle to save power. Soon all the major chip manufacturers started copying them.