Not quite. You don't give your gubmint enough credit:The EPA estimate is at 55MPH - on the Mass Turnpike 80MPH is the norm and 15 mpg sounds right.
Detailed Test Information
EPA tests vehicles by running them through a series of driving routines, or schedules, that specify vehicle speed for each point in time during the laboratory tests.
Also, they updated their testing criteria several times over the years, so you can't compare your MPG on your current vehicle ratings to the ratings on a vehicle that's a decade or so older.