"New Posts" Tab - When do Threads Appear?

[Winston]

We wonder what determines whether a thread appears, chronologically, under the New Posts tab. We thought the answer clear and obvious - - all threads appear in reverse chronological order according to their most recent posts.

But yesterday we looked to see if there had been any further postings to a thread in which we had been a recent participant, the "To Reserve or not" thread, and the thread did not appear under the "New Posts" tab despite that recent activity.

The "New Posts" tab has been a useful tool to quickly update on Forum activity and we would like to understand any exceptions to the chronological rule we thought it followed.