The math is fine but you are making a massive assumption: That every exotic armor piece is weighted the same by the supposed RNG. Planet Destiny used the Xur bug to look back at his previous inventory and they saw only one appearance of Skull of Dire Ahakwhatever. 1/52 times Skull popped up, which suggests that certain items are weighted more heavily then others. Sunbreakers showing up time after time after time and now Voidfang popping up over and over and over means one of a couple things: we have been screwed by RNGesus, some items are weighted considerably more then others or the RNG of this game sticks, horribly. We just do not have enough data to assume an even weight of all the items so unfortunately your proof is only proof of your assumptions.