I think this may have to do with your screen's resolution (what resolution do you have?). If comments are enabled the whole section is probably too wide to notice whether it is centred or not. I can't think of any elegant alternative other than allowing comments to be disabled. That makes the section narrower, which makes it clear when it is centred. See the attached image: the top is the result with centerd disabled, the bottom is the result with centred enabled on a 1600x900 screen.
If this is not how it's being displayed on your Moodle installation, you may not be running the latest version, see my last post for the link.