Number one: if you spend a bit of time in a thread reading it, Discourse sets the status for the thread to tracking (default status is only notifications for mentions).
Once you post, it will have you get specific notifications for mentions/direct responses to you.
Number two is exactly that. And three the same as one again. You can see (and/or change) the status for every topic at the bottom of a thread. The message next to the status also mentions why it is like that.
There is definitely something off with the automatic tracking because I've seen some random markers over the last few months. Hard to pin down though and I really don't have the time to do an analysis of it.