It boils down mostly to less overhead losses. The car's systems pulls around 600 watts when the high voltage circuit is active. This is near half the power input of a level 1 charge, the other half actually making it into the pack. During a level 2 charge, you still have the 600 watt overhead, but instead of 600-700 watts going to the pack, you now have over 2 kW going in.
Charge times at various power levels:
120 volt, 6 amps - practically forever (near zero measured power at the pack)
120 volt, 8 amps - 22 hours
120 volt, 12 amps - 14 hours
240 volt, 12 amps - 4.5 hours
The faster the pack finishes its charge, the less time the overhead consumption is active, the less energy required to charge the pack. Both of my cars have always charged around 3 bars/hour on level 2 and around 1 bar/hour on level 1.
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
Solar-powered since 10/10/2013
"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015
2017 Bolt EV LT in Orange with QC - 7/31/2017
Driving electric since 2-21-2013.