If the URL matches (or is "similar enough" under some secret metric) then a submission acts as an upvote for the original. In this case the URL is no "similar enough" so it's a new submission.
What I'd really like to see is for this behavior to also give a link a second chance on the New page. As it stands, if someone submits something, it falls off New with no upvotes, adding a vote two days later is completely useless.