Question about the range calculator

Hello V8 community,

I find the calculation of this computer a bit strange. Maybe it doesn't work right with me either. Example: If I can drive 70 kilometers at the calculated tank capacity and then drive a long time federal road at constant speed, the calculator calculates up to 20 kilometers more range. Gradually he goes to 80 and then to 90. But if I have to slow down and speed up once or twice (of course no kick-down), it does not count from 90 backwards, but goes back to 70 and even more down. In this sense, a powerful consumption should have emerged in the short term. If I continue constantly, he often goes up again at 10. Is the computer correct? Is there anything you can do? At some point the L. How many liters should still be in there? If I go to L's, I'll never get over 69 liters. With 80 liters of capacity there must be plenty in it. Or do I think something wrong? Who can tell me?

Greeting Manfred