This happened to me as well. Best part? It got nerfed the next week >.>
It does seem as if the "new" mythics have a higher chance than the old ones. Also, it would indeed be nice with some sort of "veto" system, as we often just end up stopping when we've had more than our share of CoEN, UK and LK.
I tried the same, we pushed 4 different keys and we all ended up with CoEN keys -.-