This is a question we get with new PowerTap users as well. The answer is the products are able to measure very small changes. In the case of the PowerTap (hub) it can measure changes in torque and velocity very rapidly. Even experienced cyclists do not have ability to put the exact same force with each leg on every pedal stroke- the result is a power reading that jumps around a lot. As cyclists improve their pedaling smoothness the power numbers fluctuate less, but the fluctuations will always exist.
For PowerCal the situation is very similar- your HR is constantly changing even though your BPM does not reflect these changes. The PowerCal picks up on even the most minute change in the time between heart beats and the result is a spike in power from a rapid change in HR, much like the PowerTap shows a rapid change in torque or velocity. The PowerCal's ability to measure and quantify these changes is what makes it such a great product and gives the user a much better experience and a more accurate measure of total work than HR alone.