There are several threads explaining the cause of the change fairly definitively. In real life, the change is probably minimal if any.

The cause was that the extra 80ish pounds curb weight punched it into the next category for test setup- different dyno resistance settings when they ran the test. Maybe 1 mpg of it could also be attributed to the PZEV (the HCH took a 1mpg hit between the regular and PZEV versions).

It would be nice to hear the exact breakout from Honda, though.