I think generally it is a factor of the watts the motor will put out in relation to rider input (at least for torque based systems). It could also be watt based - really they are kind of meaningless, and just really a scale against the power output of the motor itself (as opposed to a measure against other motors).
It's amazing how fast wind resistance increases with speed, and how much power it takes to overcome it.
I guess I fit right in with your average rider as I average 12-15 MPH on the level, between 100-200 watts. I don't (95% of the time) use my assist (or throttle) to go faster that, but I don't slow down with wind gusts, or hills. I wanted my E-bike to extend my range, and help on hills. I's working out great, just as I had expected / hoped. With this type of riding I do get the range as claimed, so far 57 KM, with some left in the tank, probably not much though.
Haven't tried, but quite sure if I went at max power the range would be considerably less.