When the activity feed is sorted by latest activity it brings activity posts with new activity to the top. but when there are many replies to the activity, it still shows old replies first so it makes it confusing why an old looking post is at the top.
For example, on my site there was a reply to a post from 5 weeks ago. the first replies to the post are from 5 and 4 weeks. because there was a new reply, it came to the top but you can only see the replies from 4-5 weeks ago and need to open it and search for what is new by looking at times posted.
If a post is brought to the top for recent activity, the recent activity should be easy to see.