The difficulty is because of the difference in operating regimes of the power trains. An old ICE has to run within an RPM range, so the top end on MPG is set b

The difficulty is because of the difference in operating regimes of the power trains. An old ICE has to run within an RPM range, so the top end on MPG is set below infinite, and the bottom end is limited by the time to ramp the engine to its max consumption. Electric vehicles have neither of these issues. On a downhill run that is long enough you could in theory fully recharge your battery, and you can always send full power (and resultant fuel consumption) to the engine. On top of this EVs are operating at much higher efficiencies so the effective size of the tank is smaller. This is not to say range estimation couldn’t be improved, only that it is a much more challenging model with EVs.