I think the explanation for this on LessWrong is the same as the explanation on Reddit (which, from what I understand, served as the code base for LW).

People don't have unlimited time, and they are willing to spend time on LW reading good posts, but unwilling to waste time reading bad posts. Thus many people will somehow filter lower posts (I do so by simply sorting the posts from highest rated to lowest rated, and read until I run out of time or get bored).

If many people do this, then posts which are "generally agreed as good" will then to shoot up as everyone votes them up. Posts which are bad will generally drop down to the filter point (usually around 0), and then stay there rather than go further below.

If everyone voted on every post, you would expected bad posts to continue dropping down, and a post which was "generally agreed as bad" would have just as large a magnitude as posts which are "generally agreed as good", except in the negative direction.

But we don't see posts with a score of -23. They seem to either be neutral, or good. So my theory that people filter to only see (and thus only vote on) neutral or better article seems to be able to predict what actually happens with the scores.