I don't know.
It could be possible because:
1. that is probably calculated using imperial gallons if in Canada (20% greater)
2. the 2500 is lighter that a full 1 ton 3500 van
3. the owner might cruise around at 50 mph (80 kph)
I doubt that 23 mpg is what most people would get. 16 or 18 mpg (US) or 20 mpg (imperial) going the speed limit on slower highways or less maybe.......... I'm just guessing.
Anyone else have an idea?