What difficulty did you killed these? And you know that they have like a 1.5% chance to get you a mythic? And this is from last season with this season blizz said in patch notes that they have adjusted the drop rates because they are not normal mythics anymore. They are now always mythics with greater affixes. They are supposed to be rare.
How much time do you spend in this game and how much time did the people you played with spend time in this game? Do they farm more efficiently?