Well this has been an amusing little discussion. I would have to say that it is a true eleven gallon pot, full right to the top.
I base this on the idea is that nobody brews or boils 11 gallons. Unless we want to assume that somebody wants to be the Spinal Tap of the brewing/boiling world.
So for practical use purposes its a 10 gallon pot with 1 gallon headroom, but marketed at its full capacity for reasons that range from marketing to truth in advertising.
I think the reason we generally don't trust container size is because of the automotive industry. Gasoline tanks in cars generally hold a little more than their official rating.
That's my take, anyway.