The Active view in the Inbox shows every conversation that still needs attention from your team. It is calculated as,
Active conversations = Total conversations - (Closed + Spam + Trash)
Why does it exclude closed conversations?
The Active view is a working list, not an archive. Once a conversation is closed, your team rarely needs to look at it day to day. If closed conversations stayed in the view, the count would keep growing over time and the view would stop being useful.
If you want to track conversations based on your own conditions, you can create a custom view.