Ok here as far as I know is the article they are linking.
http://www.popularmechanics.com/cars/alternative-fuel/electric/chevy-volt-range-tests
"In addition to measuring EV range, we also recorded the fuel use when the car was in its charge sustaining" mode. In other words, we computed the fuel economy after the battery was depleted, both on our city loop and the highway trip. In the city, we recorded 31.67 mpg and achieved 36.0 mpg on the highway. If we factor in the distance traveled on the battery's energy the fuel economy jumps to 37.5 mpg city and 38.15 mpg highway."
Looks like the numbers were taken on a long trip, they mention a 600 mile trip earlier in the article but I dont know if this is where they got the numbers from. Most of the rest of the article they talk about how long of a charge they get just from the battery about 33 miles. The idea with the volt is that for your average commute you should be mostly using the battery so over short distances it should work out very well. I dont think anyone will argue that it is going to do really well over long distances and that will be a weak point of any hybrid or electric vehicle.