I have long suspected that this is true for certain types of articles. If a new article appears on some really popular website like CNN or Slate (for example)

I have long suspected that this is true for certain types of articles. If a new article appears on some really popular website like CNN or Slate (for example) and it is about some HN-worthy topic, there's a good chance that multiple HN users will encounter the article browsing the original source site and submit it from there. Now you've got a submission with 3 or 4 upvotes, none of them from the /new page.