Yes, that is because more posts have come in since my last deletion. If the problem still exists, if I go in and delete the latest post(s) on the first thread (which I don't want to do now), then the same post remains at the top of the page, even though a more recent post is below it.
So I just did a test and posted a message to a thread from last year, then went in and deleted it. You can see now that the thread from 2017 is at the top of the page.