2 09 Aug 2015 20:55
1) Most of the time that is when someone replies.
Exceptionally, when someone edits a topic or reply, even for a small typo.
The most notable instance of this side effect is when a moderator moves a misplaced thread into the correct forum... It's kind of technically correct that the thread has changed since you last read it, but maybe most of the time, you don't care about that kind of change... right? ;)
2) Yes, I found that missing too on a couple of occasions.