I'm not sure why it's happening on your forums, but on my local copy of miniBB, as well on our live forums and
test forums I can't repeat the same. I also see that the function which gets numbers of posts in those lists, use the same value from the database, so it's difficult to understand where the discrepancy happens.
Make sure you have the latest version of bb_admin.php file as it's presented in 2.2a.