I think in this case the question is one we keep asking, but can never come up with an answer for. What is a legitimate poster? If the studio authorizes printing a gazillion extra copies beyond what is needed so they can be distributed to the fan market, are they legitimate posters? What if the movie becomes successful that the studio authorizes a second, third, etc run? What if the printer runs off a few thousand extra and 'forgets' to give them to the studio? Should we only count theater hung material? Folks, don't bother to start the debate again, we already know there is no real answer here. This is one reason (of many) why I mostly collect pre-1980.
I don't think there was any need for fakes on Avatar because everybody expected a big demand and they printed a large number of them.