Gas mileage not accurate?

So, this is not a really big deal, however, I was curious as to why my vehicle's gas mileage on my dashboard never reads accurately. For instance, if I put gas in and fill it to full, it is most likely going to show a reading around 360 miles worth of gas, even though the gas needle if full to the right. This is not correct. I noticed that if I reset the battery, the gas mileage on my dashboard will then read correctly at around 480-500 miles if filled full.

Like I said, it's not really a big deal, because as long as the gas needle is ready accurately then I'm okay with it. I was just curious about why the reading is always off. Does this have something to do with a fuel gauge sensor, or maybe the fuse?