Ok, my guess...
1)
https://wiki.simplemachines.org/smf/How_to_view_unread_posts_and_replies"unread since" is probably: unread since the last visit (log-off?)
2) If it is topics ...
I know the page says "posts" in some cases...
If it is topics...
has an image, showing 3 out of 4 entries.
The "topic" that is *not* shown, has the "post" with the most recent date (the newest post).
If the 3 other posts fall under "since last log off", then why not this?
Well, if it is topics.
Lets call that topic "A".
If "A" had an earlier post "a(5)" that had been unread (and in turn made "A" unread) *before* the last log-off, then "A" had been unread (and shown as such) at that time.
"A" therefore has *not* become unread since the last log-off. Hence "A" is not on the list. Even though by now the topic "A" has new posts "A(7)", and that post happened after the "seen since last log-off".
So by that rule, it could be explained. Now, I don't know how it really is done.... So this is just one theory.
And, yes, reading the linked doc => I would expect it to be different "opics which have had new posts made in them since the user's last visit".
- Any new post, regardless of pre-existing unread posts.
- But it could be "changed from having no new posts, to having new posts"
Btw, the source of smf can be downloaded. So it can be found out how it works....