Regarding these ranking comments...do they use a standard convention? eg They're wordpress comments, drupal comments, etc...and have the classes/ids typically expected to accompany these types of comments?
Big question is how does google know this is a comment? Cutts was asked about acceptable nav-bars once, and he made the comment that as long as you stick with one of the major ones, you'll probably fine, which to me suggest google profiles some of the more popular CMS'es and may know based on the markup profile that a comment is a comment.
I suppose the other thing is perhaps google tracks the age of different sections of a page. So if the top section never changes...but additional stuff gets added at the bottom it may be considered more fresh and trump the content at the top.