To add to
@ZeeOSix and
@TiGeo , there have been several tanks on our '18 CRV where the calculated MPG was higher, *OR* lower than the dash MPG readout.
For some reason, our GM is WAY off, to the negative. The BMW is exact to about 0.1, and the Lexus is exact to about 0.3.
To me, this indicates that they way they are metering/measuring the fuel is different. Someone out there knows why and how it's done.
And my buddy said the same thing about his Fords, way off.
It all goes back to high school stats class--reliable, and valid.
In a way it's like asking why the "range" on a car is so inaccurate, despite car manufacturers consulting the world's top fortune tellers and card readers, with astrological horoscopes baked in to the algorithms.
The mystery of the original question will never be answered, nor will the Loch Ness monster ever sign autographs...
(one of our earliest lessons in our time, occurred at age 5 or so, being baseball fans and calculating batting averages--it was a lesson in applied math that maybe we more or less observed and did, without fully understanding until we were 8, maybe 9. Seems to be the same with mpgs in the modern era, difference being we had to calculate long hand, and we were kids)