I can't speak to specific cases because of confidentiality issues, but in general, quite a few of the people you see listed as "banned" are duplicate accounts for banned members.
It takes quite a bit to get banned here - if you have a look at our rules page, you can see the normal procedure. However, once a person is banned, they stay banned, so if they open up a new account, it gets banned immediately as well.
As a ballpark guess, I'd say that for every 10 accounts that show up as "banned", 8 or 9 of them are duplicate accounts for previously banned users.
Something else to keep in mind: I'm not sure of other forums' policies, but I suspect that some of them might delete all of a user's posts when they're banned. Typically, we'll only delete those posts that break the rules themselves. Also, I'm not sure how many forums make it as obvious as RF does that a user has been banned.