If your driving pattern and style are fairly consistent, that is quite a gap. Obviously there are other conditions such as weather (wind and temperature) that can cause variances. I have read that avg. mpg on many vehicles is incorrect as it seems the technology is just not there yet to accurately accommodate all the variances. If your speedo is calibrated it is best to hand calculate rather than rely on the lie-o-meter. I just got my truck calibrated and on the first tank avg. mpg was 17.1, but hand calc was 15. I will be hand calculating moving forward as I play with the different tune options and continue to add my final exhaust upgrades. I would suggest you hand calc to see if you are really experiencing a gap. Had a bad misfire on my last dodge, but the mis was reducing my mpg by 1. However, I wasn't hand calculating, so who really knows..
