I had similar problem with my FX. Gas nozzles have a sensor that is supposed to stop feuling once the tank is full and there is back pressure generated from the tank. Sometime if the fuel nozzle is very fitted to the gas tank opening, the gas fumes can generate enough back pressure to stop the nozzle before the tank is full. At least that was the theory I came up with. Now, I simply wiggle the nozzle and fill once more right after it stops automatically just to make sure it is full!!

Hope that makes sense and is helpful!