As explained in the Welcome topic, some categories are muted by default (both for anonymous users and users who have an account):
Users who have an account can go to their user preferences under the Categories tab and customize their muted categories if they want to.
Individual topics also have their own notification settings so that explains why you still see some #off-topic topics on your latest view. This is because you are either tracking or watching these particular topics. Click on the topic and scroll to the bottom to see your notification settings for that topic.