Welcome to the forum! Those numbers the computer shows you were not meant to be taken as hard numbers. They are provided to give you a tool to use in gauging the effects of you driving and its immediate results on mpg, if you want to use it to change your driving habits. I played with this for a bit when I got my '07, but I soon learned (not that I didn't already know it) that it did not change my driving habits. I am a leadfoot, period. I actually think the feature is a safety hazard, and should not be there, as it just gives OCD drivers another serious distraction to divert their focus from paying full attention to their driving. The truth is, most people will drive like they are used to driving anyway, and would be well advised to just do that as opposed to being constantly distracted by focusing on what mileage they are getting, which is not going to do you much good after you look down at the display and then plow into the back of the vehicle in front of you. Being a retired truck driver, driver distraction is a pet peeve of mine, after long years of seeing the death and destruction it causes. Just call me sunshine!
I have found over time that the only really accurate way to measure fuel mileage is tracking it manually, with a pad and pencil and calculator at fill-up time. If I had a question about it all it would be this: Why is it that, though the car is computer equipped, it does not have a feature for this purpose? But I already know the answer, too. People don't really want it, they just want a toy to play with. So that's what they give them.