Second a website might change the title, especially for fast developing stories. I had a mod chew me out once for changing the title when if you looked at the URL the original title was still embedded in it.
Sometimes I search for a news story and find it posted from a different source. Then I've got to decide if the story I found contains enough new information to make it worth reposting.
Last of course people are just lazy. If you found a story about where Palin ate for lunch chances are it has already been posted twelve times from seven different sources in Bloggers, Vanity, News/Current Events, Front Page and Religion. You need to check.
If you are posting a story about Burkina Faso or Tajikistan you probably don't need to search as closely.
I think it’s the “lastly” one.