From your description, you're doing genuine P&G. And based on that, I'm not at all surprised to see 55.9 mpg vs. 39.
When
I compared engine-off coasting vs. "normal" conservative driving in my car on an urban loop, I saw a 12.9% gain. My fuel economy went from 50 mpg to 63 mpg.
Knowing what I do now, had I repeated that test loop a third time using dedicated P&G rather than just "opportunity coasting" EOC, I would have seen numbers in the 75-90 mpg range.
Normally I would question the effectiveness of P&G at the very high freeway speeds you cite (80-60 mph), and I'd want to see the results on instrumentation at the time rather than just at the pump. That said, if there are hills involved, and if you're benefitting from the "corridor effect" of air movement within a stream of traffic, those speeds may just be working for you.