I'm of the opinion that unless you have perfectly installed bladders you're never going to get the full load specified in the manual, and I carry that as far as thinking you'll never get perfectly installed bladders for numerous reasons. If you consider how they are installed and what they actually are, I bet only a hand full of planes actually got the listed amount of gas when filled.
My airplane has 33.5gal aux tanks and if I fill them completely to the top I get 30gal even in both of them. I still don't know exactly how much fits into my main tank and hoping to figure that out soon. I can guarantee it is not 156gal though. :-)