When 2 messages posted one after another have identical title, there is no advantage in repeating the exact same title again and again.
Some threads have over 50 messages and 99% of the messages have identical title.
Only when title is identical, the change is implemented.
Few of the words from the message body are taken and are used as new title.
That gives far more information to anyone interested in that thread, compared to repeating 50 identical titles.
If you change title of your message, and your title is uniq, it will not be automatically replaced.