This is a longstanding issue with pprune and I guess it won't be getting changed anytime soon. I might guess that it is caused by incorrectly accounting (not at all:-) for deleted posts. All long threads seem to suffer from this. I just don't worry about it.
The only practical problem, which may be related, is that the post numbers change as posts are deleted (in some threads the moderators delete quite a few posts) and so they cannot be relied upon for references. The "(permalink)" link at the top right of a post seems to work fine and I would suggest that it should be used when referring to posts.
Last edited by jimjim1; 2nd Sep 2020 at 20:26.