Battery charger

I notice that the iPod case is marked "Rated 5-30Vdc 1A" while the companying charger is marked "Rated 12Vdc 0.67A". The iPod can also be charged using a USB port where normally provides 5Vdc 0.5A. How come the charger have such high output voltage and both charger and USB supply much lower current to the required 1A? Your guidance is appreciated.