It doesn't and isn't intended to. The PowerTap directly measures power through strain gauges on the torque tube, which is a direct measurement of power. PowerCal is an estimated value derived from HR.
How, after all these years of saying HR alone is not the best measurement for training, can you now measure power from HR?
There has always been a correlation between HR and power, it's just that it varies depending on conditions. We've simply designed algorithms which predict power based on HR.