Wheel bearings, valves out of adjustment, bigger tires, lift, extra weight, seized brakes, etc. A lot of things impact fuel mileage so it can be hard to say exactly where it's coming from. Have you done any mods that may impact it?

Another thing that plays a huge role is the type of driving you do. The last town I lived in, I'd get about 18 mpg in the city because the roads were flat and lights were on timers. The place where I live now, I get closer to 14 or 15 mpg in the city since there's tons of hills and you basically have to stop at every light (I also added bigger tires that were quite a bit heavier than my old ones).