That's more distorting effects - ANH was an epic movie of hitherto unseen proportions, it was something new, and that generated hype in and of itself. To say "ESB made more money therefore it's the better movie" is misleading in that context.
But that's exactly what I'm saying. While revenue grossly correlates to popularity, that popularity isn't automatically reflective of product quality because there are so many distorting factors. Marketing, hype, maybe something special like a star casting or a scandal or whatever. There's a number of things influencing revenue results, which is why movie investment is still a bit of an uncertain animal.