Yes, that’s been noted before. Luckily, it’s not important, it’s just wrong. I thought it was reported at trac, but I can’t find it right now: https://trac.bbpress.org/
If it’s happening at your forum, try a recount in the admin.
I did talk with sambauers in June, and the problem existed in 0.9. bbpress.org is using 1.0 now, so I’m not sure if it’s the same problem or a different one.
