I listened to a Q&A with Luke Smith last night that must have been recorded sometime last week. He spoke of how annoyed people were that they were getting the same items from exotic engrams and about getting the invective 3 times in a row. He spoke about the armours saying Titans and Hunters only have one exotic chest that can drop from engrams right now and that's the Alpha Lupi. I don't think this chest is meant to drop from an engram which is why you and one other guy are the rarity that have it.