That one I haven't figured out but have you noticed how exotics will drop like mad for everyone one week and then the next week is dry as heck. Last week my raid team I run with we got between death singer and Crota normals and hard atheons and gorgon chest and nightfalls a total of 29 exotics but this week after doing the same amount we only got 8 as a six man team. So I kind of wonder if they do mess with the rng each week