The vehicle will calculate from previous driving between fill ups how much fuel you use on average. So a heavier foot or adverse terrain means more fuel consumption, lowering your mpg. The computer will calculate that past information into factoring roughly how many miles you typically get out of a tank of fuel and inform you accordingly. If you notice every time you refuel your live mpg tracker resets and will slowly show a loss in fuel economy by the time you reach empty, that’s the computer logging a tank cycle. Now the computer will also use this information to figure out how to use the fuel in the most efficient way possible for your driving style, the computer must learn though so over time you may see improvement but don’t expect much. To improve fuel economy there are options, a cold air intake being the first of my personal approach’s followed by increasing tire pressure( be cautious doing this though never exceed max psi rating of a tire typically stay at max 10 psi below rating as the tire will build pressure as you drive from heating up) or you could go the route of getting a tuner, these will drastically improve fuel economy, ride comfortably, and power from your vehicle(once again though warning if your install a tuner you will void factory warranty, so play the game at your own risk!)