Here's a theory, based on what I know about my own site
It's been around a long time, gone through a number of major IPB/IPS releasess.
forums_posts has 3,328,619 rows
The latest ID is 4,531,883
My forums statistics says 3,328,131
Which one is correctly stating how many posts have been made? I can 100% state that the latest ID is correct - that is exactly how many posts have been created in that table since the inception of the site.
Why the different numbers?
1) Pre-4.x, my total posts displayed in the equivalent widget on IPS 3.4.x was somewhere in the region of ~3,500,000
2) Over the years, some other admins were really into the whole pruning functionality to "keep the database optimised"
3) Back in 2006, database was corrupted and we lost a stack of posts and topics - I recovered enough of them to continue but lost a good chunk.
4) IPB pre-4.x had a "Recount and rebuild" functionality which allowed the number of posts on the forum to be recounted to what is what at that time, as opposed to a count of what has been done over time. It wasn't the total number of posts at the time, unless you forced it to be recounted...which I'd done a few times.
5) IPS 4.x now has the 'Forums Statistics' widget, and it counts the number as:
- the number of records in forums_posts that are not queued plus
- any posts that have been archived, if archiving is enabled (see 'forums_archive_posts') and are also not queued.
For me, the real "what is the 1,000,000th post?" is answered by:
select * from forums_posts where pid = 1000000 (Well, it would be if it hadn't been deleted/lost over the years)