Note: “Previous” and “Next” in this case refer to posts in the order that they are in, not to any particular direction in time. This often confuses many people, as WordPress, by default displays posts starting from the newest and proceeding backwards in time. Using this default ordering, “Next” would be moving backwards in time, because the “Next” page after page 1 would be page 2, and that would move to older posts. If the post ordering is changed (like via a manual usage of query_posts in a template), then the links will point in different directions. This codex article uses both methods without explanation, because it is example code only. So it is important to keep in mind that the function is referring to an order that is independent of chronological time.