Ohm's Law: http://en.m.wikipedia.org/wiki/Ohm's_Law
http://en.m.wikipedia.org/wiki/Volt
That tells us what the 5v x 1 or 2A translates to. It would indicate that twice the wattage is coming out of the iPad charger (W=V*A) and twice the current (I=V/R).
As I understand it, the block acts as a transformer: http://en.m.wikipedia.org/wiki/Transformer
So, it takes the 120V*15A=1800W power from the outlet and converts that power so that it will be transmitted at a much lower rate, giving the device less of that wattage.
I'm not sure if the earlier post by someone was trying to say that the iPhone has a particular resistor of it's own and so any different level and rate of power would simply dissipate, but wouldn't that still be stressful on that resistor?
I think, in the end, one would be better off spending the couple of bucks for an iPhone charger.
If there was absolutely no difference, then why would they make different chargers with different values? There would only be a need for stronger chargers for larger, more powerful devices, but the inverse (using that more powerful charger for a smaller, less powerful device) wouldn't make any sense.
Just use the corresponding charger.
In theory, the charger will take in the AC voltage (120V AC) and convert into DC using rectifiers. The DC voltage will then pass through a voltage regulator to maintain the output voltage at 5V (or whatever voltage is desired). The voltage regulator, in basic electrical engineering, it is just a simple circuit involving resistors and zener diode. The zener diode's job is to drain the excess voltage and current to ground. Unlike human, electronics component will just keep working on and on and will not be stressful.
Some devices require higher voltage than other. There are many reasons that could contribute to that differences, which I am not exactly sure. However, some of the reason that might contribute to this are:
(1) the input impedance of the devices might be high or low
(2) poor circuits design that require higher voltage and current but does not use to contribute to the actual need of the equipment
(3) the engineer who design the equipment said "I like it that way"
***hope it helps***