Because the construction of the loading device influence the output of the generator related to speed.

1. If one device get a input of 3W at 20 km/h and have a efficiency of 80 percent and the other device get a input of 5W at 20 km/h and have a efficiency of 75 percent, than the second device is the better device, in spite of lower efficiency.

2. The same using rechargeable battery. It may lower efficiency. But if your navigation device battery is full and the loading device produce more energy than the navigation device needs, you can save energy for the times, when the loading device produce too little energy. So with a rechargeable battery and lower efficiency you can produce more useable energy than without a rechargeable battery and higher efficiency.