It could also be the position of the float arm in the tank as well. If it was bent at some point it would not read accuratly. Think of it like the float arm in your toilet. If its set to high, the water will reach the over flow and never shut off. If its to low, you don't get a powerfull flush. Or relating to the jeep, either you run out of gas at 1/4 of a tank, or your sitting on E with 1/4 tank left.
What have we got?
We have one heavily armed Recreational Vehicle
Known issue in the TJ's. Best thing to do is not worry about. Just reset your trip odometer everytime you get gas. That way you'll know how many miles you have traveled since fill up. Just start to think about refilling when you get around 200-250 miles, according to what your particular set up is. My light always comes on around 180-200 miles even though I still have 4+ gallons in it.....not a big deal to worry about that gauge to me.
Not trying to open a new can of worms but...all TJs have the same (19gal) tank. The so-called 15 gal. ones have a longer vent tube that prevents the last 4 gal. from entering the tank. There are write ups on how to cut the tube & gain the extra capacity (don't cut the wrong tube). But to the point, there are 2 different sending units. One for 15 & one for 19. The 15 reads full when about 3/4 full in a 19 tank. I would guess you have a 19 gal sending unit in your tank. When you fill it to 15 gal (full to you) the sender only goes up 3/4 of the way. Don't know how or if that affects the warning light, they all seem to vary a lot. I found a photo of a sender somewhere & you can see the label says TJ15 which I think is from a 15 gal tank. Some that cut the vent then switch to the 19 sender, others live with the fact that they use 1/4 tank before the gauge begins to drop.