The Powertap just guesses cadence from pulses in the power- if you want the real figure use a specific cadence sensor/cadence wheel speed sensor alongside the Powertap.
I did try to - but Garmin didn't want to play with the cadence sensor for some reason and I didn't want to get off and start mucking about with the bike in the middle of the session.
The Powertap just guesses cadence from pulses in the power- if you want the real figure use a specific cadence sensor/cadence wheel speed sensor alongside the Powertap.