Once you have banned them and the user count has gone down, placing them in a user group (whatever the permissions) does not make the user count go up again.
I'm saying that they are never officially banned.
Instead, you see this a lot with
big boards (1m+ members). If you go
community/members/mr-lucky.########/
, you can still see their profiles with 0 posts.
Some profiles have usernames like viagraforsale (or another known spam product) that make things a bit more obvious what they're doing.
What they're doing is creating a new usergroup BANNED (or whatever they call it) that has permissions set to NEVER (can't post, reply, react, etc.) and adding spammers to that group.
This retains the "BANNED" member as a "valid" countable member in Forum statistics.
They aren't banned, per se, but they can't do anything on the forum other than login and log out, which is essentially being banned.
Don't believe me? Edit the number and visit profiles on big boards using XenForo (start at about 1/2 way, so if 1 million, start at 500000, like
community/members/mr-lucky.########/
, and count the suspicious "active members" going one by one and you might see an obvious spam account every 10 or so) to see member profiles and tell me they're not "banning" them to keep user counts high.
They're not being banned through the traditional ban system, rather just being assigned to a "banned" usergroup to inflate numbers.