Why is a cellphone charger rated like that?
It has more to with how switching supplies are built. They rectify the mains and smooth it with a capacitor. Problem is, current can only flow when the input voltage is higher than the capacitor voltage (peak of the sine wave). So what happens is that the power supply draws energy from the mains in short bursts (while the capacitor feeds a steady trickle of power to the rest of the power supply), so it may very well be pulling 200mA, but only 25% of the time. For small supplies up to a hundred watts or so this is no big deal, but in larger units steps need to be taken to make sure the power supply draws a more continuous stream of power rather than in intense bursts (Power factor Correction). (side note, cheap electronics sometimes skip these steps, there was a no-name led light I tested at my old job that pulled all it's energy in a brief 200W burst despite only being rated to 5W, it hit 135'C on the outside amazingly it kept going). Most power supplies that cost more than a dollar perform better than that particular led.