It doesn't 'create' a universe per se. Every variable already exists in the multiverse. When you go back, you are just going back to the version where you went back. You've always been there. So because the picture of Bell changed for the characters who went back, DS9 from that point forward is set in a different universe to the one where those characters started. If Dax had whipped out the photo and it was Sisko all along, they would be in the same universe where they started.