Why is the "In reply to" indicator suppressed sometimes?

thanks, Antinous.
@codinghorror i know what i said above, but I totally randomly just found this example in an old thread, the guy says he’s confused for the same reasons we are:
http://bbs.boingboing.net/t/tip-create-an-account-at-meta-discourse-org-edit-or-not-your-choice/146/7?u=noahdjango