That happens to me too. It all goes away at the first stop light. You have no idea, or you do, how much sitting still ruins that calculations.
I find I can cheat the calc a little just by coasting up to a light and/or breaking to a really slow speed trying my best to reduce the amount of time standing still.
It works. I find it odd that if I put it in neutral, I have a manual, and coast and slow by breaking, etc., it thinks I have better economy but in reality, if I am idling while coast or sitting at the light, I still travel the same distance and burned the same amount of fuel.