It does sound odd but I've seen and heard of more bizarre things. I built a 2bbl on a '86 5th Avenue 318 and set it up with a 4 gas analyzer and did a major tune-up. The owner drove it to Texas from SC and got 27 mpg. He showed me the gas/mileage log to prove it. I'd never heard of another that did that, they only average 18 mpg tops on a good day. He was so impressed he wrote the dealership and Chrysler about it. Go figure! A 50% increase!!! HOLY SMOKES..........
I thought I was doing good getting 20 mpg in my '69 on the highway. (taking it easy of course with 100% gas)

 

There are smart phone apps that could either confirm your results (by mapping your drive), or completely debunk them. I would be curious to see the hard evidence here.