If you set everything to 0 and fill your tank it wil give you a predicted milage range based on your last spell of driving (its clever you see).
If you have just been driving steady on the motorway and achieving lets say 35mpg, when you fill up it will say a predicted range of lets say 400miles. You will only achieve the 400 miles if you drive like you are frighten of killing flies.
If you have been on track all day (killing flies) and fill the tank it will say that you will have around 200 miles to your tank.
your car is constantly calculating your range based on your mpg. Less mpg and your range will fall, higher mpg and your range will increase.