it is a networking thing, not everyone has adopted IPV6 yet (in fact according to google only 26% of the world is using it right now) but it is recommended to move to V6 because of V4 exhaustion, so if you have it you must advertise that but to let those who don't have it you must also advertise V4.
i believe what bitcoin does is that it mapps all IP addresses to V6 so it can show you both at the same time.