This is only a guess since no one else has posted i thought i would take a shot at it.
A lean mix is when the engine is getting more air then it needs and less fuel then it needs, with this logic we can say that the fuel is more 'vaporized' and would thus combust quicker but produce less power. This would cause you to idle high because its combusting faster due to the increase in air and less fuel to burn. Needing to do so at an increased rate to keep the required power, since less is produced.
On the other hand a rich mixture would have less air and more fuel which requires a longer combustion to occur (providing peak power, anything past peak wouldn't be burned and would be wasted).
I think in each case fuel and air, when mixed, can only ignite in a certain range, so once you get that range any excess fuel or air gets wasted. As you have less fuel to ignite it burns faster, and as you have less air it burns slower.
Imagine that you have a bottle full of gas and a campfire. If you spray the gas on the fire in a fine mist (lean) it will explode fairly fast and die out fairly fast. If you take a burning stick and just drunk it in the bottle it will either put out the flames (due to lack of oxygen) or just ignite, but without an explosion. Thats my idea of lean and rich.
