Just to add to the confusion, sometimes the labelling itself is misleading: a couple of my chargers are marked 1.1A and 2A, but in fact they'll each supply 2.5A to a suitably dumb device. Internally, they're configured so that one port will present itself as a 1.1A charger, and the other will present itself as a 2.0A charger, to the appropriate Apple device.
Finally, I have USB cables that are internally set to do the same thing with any charger.
It took a cheap USB current monitor to work out what the heck was going on in my growing collection of chargers.