I'm not sure if this is truly rare or not. The add says the Astro was special ordered with a 3/4 ton frame.
https://cosprings.craigslist.org/cto/5420150995.html
As someone with a deep love for these vans, over two decade of ownership of them and countless hours of wrenching on them, the ad is incorrect.
They never came with a 3/4 ton frame, an additional leaf for the rear spring pack was available which did increase the load capacity by 250lbs. My van Grumpy has this extra leaf.
Also, they typically do not average over 20 mpg in town. They might be looking at the overhead info center after they get up to speed and see the current mpg displaying over 20 mpg. I hypermile my vehicles and my previous 2WD was just shy of 20 mpg on average Chicago suburban driving. Getting it up that high takes more effort and thought than most modern drivers want to deal with, so I highly doubt it's their average.
You may review my fuel logs at the link below. The 2WD Astro is under my retired vehicles under the name Turf.
http://www.fuelly.com/driver/ihatemybike