I can think of several reasons, but am not quite sure what you're asking, so let me guess and you can correct me if I'm wrong.
You look at the list of Recent Postings and you see a thread. Out in the right hand side it shows that SirArrrgggh commented on the thread at 2:15 today. You open the thread and the first thing you see is a posting by
@SoggyShorts which is dated November 2, 2018 at 7:45. Right?
The answer seems to be that it opens the thread at the last posting that you have seen (or the beginning of the thread if you haven't looked at it). If you go down to the bottom of the screen and page up to [3] (or whatever number), you will find the posting by SirArrrgggh. This is based on my observations, not any knowledge of the Forum coding.