It is easily possible to get more than the rated output out of a dynamo hub. Dynamo hubs are constant current devices that produce AC over an increasing voltage range as speed increases. By matching the load to the output voltage using capacitors and resistors the output can be much greater than the rated output. As an example the Forumslader has 5 stage load matching, which is switched by a microprocessor, plus a voltage booster at low speeds to maximise output. I suspect some of the more recent chargers have the same thing, given their high outputs.