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That means that your truck was optioned with the 32 gallon tank from factory, the majority of the left hand side column is what the truck comes preplanned with, and the rest of the options listed under the "optional equipment" is what got selected to be added on. So technically yes the original customer bought the 32 gallon tank, but it was through the dealer factory-ordering the truck when it was being purchased.
They have been known to have about a 5-6 gallon reserve so I doubt if you'll ever put in more than 20 something depending on how far you drive on empty. In other words, the gauge ain't very accurate.
Jay