Drupal works off a "history" table to keep track of the content you have looked at (in the last 30 days). When you import old content that users have seen already it's shown as "new" because there is no entry in the history table yet.
I could insert records into the history table for all users and all content. I remember someone posted on d.o. about a "trick" that doesn't require inserting records into history, but I can't find the reference now.