I was playing Anniversary BTB and me an my friend were on the map Ridgeline. We were playing a CTF gametype and as we both returned a flag we got an achievement called “Return To Sender”, which is a Defiant achievement for returning two flags in a matchmade game. Neither me nor him have purchased the Defiant map pack, I only have Noble and Anniversary, and he only has anniversary. Can someone explain to me why this happened?
Anyone? 
The earlier batch of Defiant achievements were not map pack specific. Return to Sender is not one of the new ones (that are map pack specific)