Right, but predictions or descriptions of human behaviour based on guesses as to where humans fall on the line between perfectly rational and completely irratio

Right, but predictions or descriptions of human behaviour based on guesses as to where humans fall on the line between perfectly rational and completely irrational are also guesses. And when discussing movie theatre popcorn I think it's pretty clear that the irrational or arbitrary aspect of human rationality is in play here. The discussion on stackexchange is silly, you mentioned anchoring effects yourself.