Someone please correct me if I'm wrong, but I believe that sharing an internet connection with someone means that you will see ads influenced by each other's network traffic, assuming you're going through the same router.
Right, it's not the only identifier, but it's one of them, and you will "pollute" each other's targeted ads to an extent. I'm assuming some sort of adtech stuff is happening at the carrier/provider level as well, independent of your browser or individual browsing device.
