Due to RNG the % of a drop rate would be 1 in total number of possible loot drops for the location. Atheon on hard you get helmet, shards, energies, VoC, AE, ship, sparrow, Vex. That would make the drop rate of the Vex Mythoclast 12.5%. Exotics are on a bonus loot table and don't factor in on your raid drops. It would also seem there is a 4th roll that is either circumstantial or has "nothing" slots on it's table. I have received 4 drops on Atheon before.