I believe the behavior is mostly as follows. If the comment is voted neutral (currently, 1 point), people only upvote or downvote if they feel strongly about whether it's good or bad (according to whatever valuation). If the comment is already upvoted or downvoted, people are more likely to cast an opposite vote if they feel that the comment is rated incorrectly (in the wrong direction), to "fix" its rating. Thus, if someone upvotes a 0-point comment, it doesn't mean that the comment is considered worthy of an upvote (to reach, say, 9 points), it only means that the person thought that it didn't deserve a downvote from 1 point.

Thus, there seems to be 2 modes of voting: hard voting and soft voting.

• Hard vote is supposed to push the comment all the way in the specified direction.
• Soft vote is supposed to push the comment in the given direction, but only towards neutral rating, and not beyond that. It's intended only to eliminate the opposing votes, not to set the vote.