My guess is the primary difference is conditions unrelated to the ethonal:
- Head wind vs tail wind can easily create that big of a difference.
- How hilly is each state (and what state did the gas actually come from)?
- What is the altitude?
- Did the engine have issues with clogged injectors or other issues from light duty use that got blown out by several days of hard use.
- Was there congestion or stop & go driving conditions?
There are issues with ethonal but the facts don't support that kind of an improvement in gas mileage just based on ethonal.