Wrong calculation. To work on high frequency the CPU currents should be increased proportionally (linear). But energy consumption increases quadratically (W = R * I * I). So, the linear increase in CPU performance is adjusted by quadruple increase in power consumption.

EDIT: well, I am not completely honest in this calculation, only part of CPU wires is a subject of that and I didn't take into account the decreased time. However, it is true - per any given number of CPU cycles the high frequency cycles take more energy.