Question about Belkin BoostCharge

I once learnt: P (energy/power) = U (voltage) * I (current) ... so how exactly is this device with an output voltage of 5V and an output current of 3A supposed to achieve an output power of 20W? Just for the record: the other (larger) models in this series (BoostCharge) also only state 15W. Is that a typo or fraud (sorry, I sometimes have problems with German)? Thanks and regards :) Päscu