Strange fuel tank problem.

Last summer, I destroyed my 20 gal fuel tank. I got a replacement from the junkyard and put it in. Since then, when it's near empty, it takes 13 gal to fill it. When I do, the gauge reads about 3/4 full. I dropped the tank and compared it to my old one. It looked like the same tank. No extra baffles or anything. My wife says she got about 18 gals in it once. I am puzzled as to why it won't take as much fuel as I think it should. Did I end up with a 13 gal and not know it?

Edit: As soon as I posted, I realized it should be in OEM tech, but I can't figure out how to delete it. I must be an idiot. Can a mod move it please?

The only difference is the vent pipe, the small tube near the fill pipe. It turns down towards the bottom of the tank and is longer in the tanks that have the less capacity (13 or 15 or whatever to many conflicting numbers). Shorten it like i did in mine and get the missing gallons back or go find a new tank.
If this tube is the same in both your tanks i would have no clue as to why the cappacities would be different.
GOJEEP has a mod on his web site that explains the difference go look at his profile and his site is listed.
Hope this helps.