2006 Chrysler Town & Country 3.3L Base Model Gas Tank Capacity?

I bought a 06' Town & Country and the gas tank is suppose to be a 20 gallon tank, but when i fill it up when it's almost on empty i can fit about 13.8 gallons? Could this mean this model van has a different thank even though on the papers its written that it comes with a 20 gallon one?