I'm not convinced that is strictly true. But I do find myself sometimes coming up with less than 5 comments on a short chapter. Then I'm faced with either trying to put them into a standard review or finding other things to highlight that aren't necessary to make the 5. That isn't really fair to the author.
What I would like to see is something like a ratio of necessary comments to length of the post. If it is under, say, 500 words, then only 2 comments, but if over 2500, then perhaps 6 or 7 comments to make the points. That way, you could still get a fixed amount of points for a post, but only have to come up with 2 to N comments.
~Tom