This is the first film to even give PB a little bit of a run for its money. Even Raiders had a tougher time than PB in every poll IIRC.
These polls depends a lot on the competition involved. And part of the problem is that - at least for me, but I believe for many people - these decisions aren't always transitive.
For example, in the previous poll, I voted for Star Wars over Empire Strikes Back. If there was another poll were I was comparing, say, Empire Strikes Back to Monty Python and the Holy Grail, I'd vote for Empire. But if I were asked to vote between Star Wars and Holy Grail, I'm pretty sure I'd vote for Grail.
And I think each of those preferences make perfect sense, though when you try to see what the overall result is, you run into a bit of a problem.