Not exactly explained by GM, but the folks over at hybridcars.com have come up with some ideas as to how GM came up with that number.
Here are the first 3:

More Than Infinity

“Under the rational that GM says the EPA applied to the Volt, an ell-electric car such as the upcoming Nissan Leaf or existing Tesla Roadster would have an official rating of ‘infinity miles per gallon.’”
- John O’Dell, Edmunds.com Green Car Advisor

Nuclear-Powered Cars

“If the Volt were powered mostly by a windmill or a nuclear reactor, it would also get great gas mileage, since the fuel would be coming from some other source.”
- Rick Newman, US News and World Report

Back on Gas

“The 230-mpg number, according to GM’s Frank Weber, global vehicle line executive for the Volt, is a measurement of the car’s ‘city-driving cycle.’ If the Volt got out on the highway—where it’s powered largely by gasoline—and traveled 200 miles, the mpg would drop like a stone.”
- Robert Chew, Time

Go to hybridcars.com for the last two.