You beat me to it.It would be immaterial if the tested speeds were identical .
MPG tests are conducted at much lower speeds, ideal for getting the highest rated MPG.something like 45 or 50.
80 mph requires a huge amount of additional energy to maintain…air resistance quadruples when speed doubles.
Not arguing, just saying I’m not surprised by the results you saw.
Speed matters most, add in variables that decrease aerodynamics = larger efficiency hit.
Additionally, the same factors that increase the work (thus the fuel economy hit), also increase stresses in the drive train, which can/should be mitigated by more rigorous maintenance.
Full disclosure, after years of racing motorcycles. I now drive "conservatively."
I await my beating behind the MUD woodshed for confessing my gas saving blasphemy.