Just for fun, I poked around a little among some of the older threads in Completed Projects, and I found this one:
viewtopic.php?f=16&t=2105 This is certainly skewing that number. For some reason, the project moved into a different thread, and then both threads were moved into Completed Projects. Maybe there was a post-per-thread limit in place at the time? I suppose there are some more examples like this scattered somewhere in there.
EDIT: Hmm. Except that I didn't actually try to navigate to the thread it moved to before posting this. I just assumed it was there.
Now I see that it is apparently nowhere, so there's that theory shot down!