The first thing that pops up in my mind is your public IPv4. You see, in your home LAN every device uses the same public IPv4 to communicate in the internet. So if one device browses for something like an iPhone and you're being tracked then those ad brokers deliver iPhone ads to this public IPv4 and every device behind this public IPv4 will see those ads. Nobody on the internet knows whether behind this public IPv4 is a single device or a LAN with many devices.