The motor power rating has negligible effect on the power consumption on flat roads.
It's a torque sensor bike, you have to input the difference between what the bike needs to move and what the battery gives. I think the range is given in optimal conditions with speed at around 12mph-13mph, no headwind and on eco setting (maximum rider's input) which is 50% assist.
The power consumption goes up as you ride faster because the air resistance is proportial to speed to the power of 3. At 15mph, the power consumption rises to 180Wh/15m=12Wh/m - the range will be thus reduced to 612/6= 102 miles.
At 28 mph, power consumption is 840Wh/28m = 30Wh/m - the range will be reduced to 612/15 = 41 miles. The power required to move the bike at 28 mph is 840W, most people can't contribute as much as half the required power (480W), so they will have to abandon the eco mode setting. At 80% assist, the range will be reduced to 612Wh/(30Wh/m*0.8)=25.5 miles.
There is negligible difference in power consumption on flat roads between different motor types (crank drive, geard hub and direct drive), the data on power consumption is from ebikes.ca.