The rate at which exotics drop is probably an algorithm tied to the number of active players in the game world in order to keep exotics from becoming common. Also like gambling the more you play the more likely you increase your chances of getting exotics. Vice versa, the gambling also means every once in a while someone will get lucky. But Bungie plays with the drop ratio for individual items by adjusting the weight of the drop. That's why No Land Beyond drops so much because it has a much higher weight than Red Death. So, the algorithm will drop more NLBs to keep the drop rate constant, but it can't drop more Ice Breakers or Gallys or Red Deaths especially as new drops. The algorithm must have a bug where if you already have an exotic it's okay to drop it more for you because it doesn't change the overall rate of new drops for number of players in the game world. That's why people tend to get more of the exotics they already have as opposed to exotics they don't. Just my opinion not based on hard statistical evidence just game play experience in other RNG games and my experience in Vegas.