Here is your answer. Multiple people post, and eventually the upvote lottery puts one of them on the front page and a discussion ensues.
Take the Nokia/Moon story I specifically referenced. Multiple people were still submitting that one the day after it first broke.
Did none of those people have the gumption to consider that, on a site, like HN, obsessed with all things technological, such a technology fan's wet dream of a story might just conceivably have already been submitted by someone else, during the preceding 24 hours?