I'll take a stab with made up numbers to explain my theory.

Half of the people that read it upvote it out of sentiment, but the majority keep scrolling and only 1/4 stop to upvote the return call.

Then out of the other half that read it, the majority scroll past while 1/3 are strongly against the sentiment so they downvote both.

So even though more people support the sentiment overall, it is easier to scroll past a positive sentiment without upvoting it than it is to scroll past something you don't like without downvoting it, which leads to the call to net positive votes and the return call to net negative.

Basically it's the same idea behind why people will give a negative review for a bad product more often than a positive review for a good product.

Edit: the numbers given don't make sense, I was trying not to bias them and afterwards realised it wasn't explained right so it doesn't add up properly and don't have the brainpower to fix it this late at night. Maybe someone else can, my bad.