That's why its an estimate, they can't get it right for all road conditions. For example here where I live the tallest hill is a hwy overpass ....
So rejoice in the fact that the numbers will always be off and the fact that we will always be able to show positve improvements on our fuel logs ..
Otherwise the sticker would have to say;
YMMVBOGCAE - Your Mileage may vary based on ground conditions and elevation