All the newer style tanks now come with a safety shut off valve to prevent overfilling, etc. The valve knob will be triangular in shape instead of the old round style. These valves have a "float" type assembly that shuts offthe flow and now usually only fills up to 1/2 gal short of the old style, sometimes more as you note above. Not too uncommon of a problem.
Bob and Better Half
2007 F-250 PSD, SW, CC, LB, 2WD
2006 TF32SS Cross Terrain Toy Hauler (sold)
Reese 16k slider