The active users panel in the ACP dashboard shows number of socket connections it is not the same data as /users?section=online
/users?section=online shows users who have performed and action in the past 30minutes(can be configured in the ACP).
The unique visitors graph shows the number of unique IPs that have visited your site per hour. It would go up whenever a new IP hits your site even if they don't have read access to any categories.