Why does the iMac charge my iPhone faster than the wall charger? I've noticed that my iMac at work (21.5", Mid 2011) charges my iPhone 5 about 2-3x faster than the included wall charger.

How does it do that?
If you have the USB 3 SuperSpeed (on your Mac) it can deliver up to 100 Watts.

For iPhone it will deliver 10 Watts (5 Volt x 2 Amps) which is more than your charger.