One of the issues is that the software allows a person to refer directly to an individual post by number.
https://www.bikeforums.net/20015918-post4.html
Deleting previous posts can not force a renumber, otherwise all the potential hyperlinks would be broken.
So, just looking at the post numbers won't give the precise page, so some other method for counting pages should be done (and apparently is being implemented in part), but isn't fully implemented. I.E. the system can jump to the proper page irrespective of deleted posts.