Deleted discussion group posts still show to people on the same subnet as the original poster. This was designed so that spammers or annoying people wouldn't actually know their post was deleted. It works very well to keep discussions on track and positive. The gadfly whose post has been deleted keeps checking back to see what the other users have said, but they magically do the mature thing and don't respond. Soon, the person who posted to the discussion to get a rise out of people (not to help the discussion) loses interest and moves on.
Note that because this is based on the original IP address of the post, it's not cookie-dependent.
If you try and test this behavior from your own network, it'll appear to you that deleted posts keep showing for all users, including anonymous viewers of the discussion, but only if the post came from your network. When you test from your own network, the behavior seems counterintuitive and confusing. But in practice, it works very well to keep discussions among people on different networks civilized and on track.