Order in which All Topics posts are displayed
Around here (compared to SMF) you need to be very clear about POSTS and TOPICS. A topic has many posts.
I think the display TOPICS list does what you describe - it displays those topics with the most recent posts at the top of the list. The forum board however refers to the most recently started TOPIC, which might not be the top of the resultant list.
The display POSTS lists displays EACH post in the order they were posted. Consequently there might be many posts for the same topic and when clicking on a POST that might not be the first unread post in that TOPIC so you are not really sure where you are entering (returning to) the TOPIC.
Personally I find all of the post listings a total waste of time, except for checking up on seeing how many posts are being made by PlusNet staff each day! 
On all but the Forum Board list, Topics that are Unread or have Unread Posts and Unread Posts, are all shown in Bold and with a red dot next to them.
On the Home Page you have modules for Recent (ie Latest) Topics, Latest Posts, Unanswered Topics, Unread Topics & Unread Posts. All are listed there with the most recent First. If you click on the > on the RHS of the module, you get a specific list of that module.
The Recent /All Topics module lists the most recently created Topic first, irrespective of what replies have been made, as well as the total number of replies to each topic & number of views.
The Latest/All Posts module lists the most recent Post first, obviously, & there maybe several posts per topic, as well as the number of replies to each specific post and the number of views of that post.
The Unanswered Topics module just lists the most recent first, with date and time of posting.
The Unread Topics module will show a Topic as Unread (bold title, it also has a "red" dot next to it) when there are posts in the Topic you haven't read. It shows the number of such New posts. If you click on that number, it will take you to the first unread post. Clicking on the Title will take you to the start of the Topic unless you've changed the setting here checking the box 'Jump to first unread post in topic', in which case it will do that, the same as clicking on the number.
The Unread Posts module is just that, newest first.
You'll see that the Unread Topics and Unread Posts are actually on the same page, with tabs to jump from one to another. You can also set Preview or Full Text. That's for the first post if viewing Topics.
The Forum Board list can display the Latest Topic and/or the Latest Post depending on the settings of the "Show the Latest Topics component"/"Show the Latest Posts component" here.
A Board listing shows the Topics listed in the order of Topic with the Newest post first - except for the Pinned Topics which seem to be Newest creation date first 
Bold title and red dots show there are Unread posts in the Topic. The listing also shows the number of Replies to a Topic and the number of views.
If you have 'Jump to the first Unread post in a topic' set (see above) clicking on the title in the Board listing will take you to that first Unread post. Here, and in the Unread Topics module are the only two places where 'Jump to the first unread post in a topic' will work.
Great explanation Anotherone 
But dont you think going through all that would be a TAD too much for a new forum poster 
More simplicity is better