When polls like these are run the numbers don't always wind up tilted in the favor of the bigger number. I wish I could provide a specific source but I've been listening to the 538 podcast for years and I know they've covered exactly this topic.

Your inability to believe a thing doesn't prevent it from being true.

I would grab a D20 and on a 16 or less I would say left otherwise I would say right. Some people would pick right just because they can. I imagine most people would pick left because it's the 80%. I imagine plenty of people would double and triple guess and waffle then say something.

Few people, even the dumbest among us, are easily modelable deterministic automata.

Because some humans don't even understand the question. Others do, but they don't know how to solve it.

But other humans can do this task. If you do it with a bunch of mathematicians in a room, you will get your 20-80 distribution.