Share 'Why do some posts get their comments closed, and not others?'

Sorry if this has been asked before, but why do some contributor posts on the front page get their comments closed, and not others?   As far as I can tell it doesn't seem to have anything to do with the age of the post, or the amount of comments posted.  Anyone know?  (and I know this is going to be super basic and obvious and when I hear it I'm going to say 'Duh!'