This filter isn't specific to polls. Anything that reaches people through reshares gets selected on how surprising or pleasing it is, so the visible sample skews toward the tails. Marketing has the same problem with screenshots of campaign results: the ones that travel are the outliers, and anyone calibrating on what they've seen shared ends up with expectations nobody can hit.

Curious whether the effect survives controlling for pollster. Some pollsters have consistent house effects and some have much bigger followings, so part of the slope could be "polls from well-known outfits get reposted more" rather than "favorable margins get reposted more".