How common are Mythic Uniques?

I had joined a clan and like every 30 min they keep posting that they have gotten another 2-3 of them. Over the last 4 hours or so I have seen multiple Grandfathers, Ring of Starless sky and Shako's. I stopped playing after completing season 1 and Unyque's were like 1 in a billion and 4 Harlequin Crest had been found and like 2 Grandfathers... are they hacking and if not how can I start farming up like 6 a day like they are?