Make sure car was level when fueling! If outside Temp was low, you need to fill more slowly to allow time for bladder to adjust to size. You usually can get at least a gallon or two more in, after pump shuts off, particularly if nozzle is too far down in tank.
If car is lower at front left the gauge could drop pre-maturally, and then recover a notch if lower at right rear. I have noticed in breaking on a wide right down-hill turn on exit ramps the gauge will sometimes drop a notch then recover when accelerating on a left turn. This leads me to believe that the gauge is not in the center of the gas tank.