the controller works by reducing your battery voltage that the motor sees via duty cycle and pulse width modulation.
the motor produces back emf voltage, which opposes the battery voltage, and which increases as the motor turns faster.
in order to maintain constant motor current, the controller has to increase the effective voltage the motor sees (and duty cycle) as the motor turns faster.
the electrical wattage is the effective voltage times the motor current, and it is also the battery voltage times the battery current.