User A posts some inappropriate content at 12:38 PM; consequently, that's when their last activity is. Moderator A notices this at 1:32 PM and deletes the content and bans the user for one week.
User A comes back online at 5:57 PM and notices they're banned. If a staff member were to look at his profile then, though, they'd still see their last activity as "Today at 12:38 PM" instead of "[Location], A moment ago."
Why is their activity not updated?