Similar to some bugs before, if you look at the post count given in the linking box of a post, it gives the post #. However, this doesn’t match the numbers displayed in the navigation bar on the right.
I think this topic may demonstrate the problem: Importing from phpBB3 - #580 by gerhard
So the last post on the topic (at the time of writing) is being called “post #580”, but but the scroll bar shows 256 / 257. Looking at the second post in the topic, it is numbered #303, so I’m guessing there are some whispers before it.
Also there is a gap in the numbers after this post: Importing from phpBB3 - #307 by Canapin
Maybe a whisper there again?