Phoenix, Arizona is actually not the most sun-soaked part of the country. The Mojave desert gets a much greater amount of solar insolation[0]. Using those numbers, we can calculate ~511 kWh/m2/year, assuming 15% efficiency as well. That lessens the gap somewhat, but there's still quite a distance - perhaps wind and hydroelectric would account for the remainder?

It may just be flat-out wrong, which is unfortunate.

[0]: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook...