The factory recommendations are not always the maximum theoretical distance you can get from your cars, those recommendations are based on real world usage, e.g. 120 km/h on a highway or 30km/h average with stops in the city center.

It is called "hypermiling", you can do that with gas cars as well, by driving on a level road, minimal breaking and accelerating, plus driving in the engine's optimal RPM period in the highest gear possible. In short, the ideal conditions for the car.