"Unread Posts" vs "Updated Topics" vs "Unread Topics"

[eto]

I usually click on "unread posts" to see what has happening since I last logged in. 

Now I recognised that "Updated Topics" or "Unread Topics" contain posts that I have not seen before. What's the logic behind these 3 things and why don't some conversations shop up on "unread posts"?

[Gryzor]

Let me look into it and I'll get back to you. I think Unread Posts has to do with topics you've been following. 

[pelrun]

Unread is going to give you every topic that has unread posts in it. Updated is only going to be those topics that you've read previously that now have new posts, or possibly that have had new posts since your last visit.

[eto]

Unread is going to give you every topic that has unread posts in it. Updated is only going to be those topics that you've read previously that now have new posts, or possibly that have had new posts since your last visit.

But how is it then possible that "unread posts" is empty but "updated topics" does show some entries? And again some others on "unread topics"?

Even the URLs don't really give a clue how they are related or different:

Unread posts: https://www.cpcwiki.eu/forum/unread/
Unread topics: https://www.cpcwiki.eu/forum/unread/?all;start=0
Updated topics: https://www.cpcwiki.eu/forum/unreadreplies/