Naively:

Everyone should agree that 100% certainty of something is infinitely overconfident. Then, everyone should upvote. Knowing this, I'm completely certain that I'll get lots of upvotes, and so absurdly large amounts of certainty seem justified. And as a kicker, everyone said I was overconfident of something that turned out to be correct.

Obviously, there are other possibilities (like me retracting the comment before it can be massively upvoted), so (like usual) 100% certainty really isn't justified. And unforseen consequences like that are exactly why you don't play with outcome pumps, as the time turner story reminds us.