It usually is, for me anyway. I made a hub about litmus paper andabout three quarters of the ads were about acids/bases so they are related to the hubs that you write. It is also to be noticed that ads are specific to your search terms on Google. If you search for dogs in Google's search engine, and then you go into a hub, some of the ads are related to 'dogs'. If you buy or search for products on Amazon or Staples for example, they will also appear on your hubs. I hope this answers your question!