Create a test user and see if the content still shows. IIRC it's to do with the recent activity being cached for the user, so it doesn't make sense updating the cache to remove it from recent activity if the user has already seen it. I believe Kier's example was if there's an error in a newspaper article - you don't ask for the newspaper back from the people who have already read the article, you just fix the error for the people who haven't read it yet.