Welcome to the site. Ford has had that problem for awhile now in certain models. I work for the railroad and we've got some F250 diesels and they have that problem. After we fill them up it will take awhile to read "FULL". I've found that when I turn the truck off and turn it back on that it will be fine. I recommend taking it to the dealer. I've heard that the newer ones have a bad sensor between the fuel take and the block but not all of them.
2007 Ford F150 Supercrew sold
2010 fully loaded Ford Fusion
2002 Mustang GT being built in garage.....stay tuned!!!!