You really have no clue how rng works. There is more then one step to how rng works and that changes the math completely. With Xur it is slightly different then the normal rng for killing a mob or end game rewards. 1 step is to see what kind of gear it will be, I.E helm, chest, gloves, primary, special, or heavy 2 step is after the gear type is set, which piece of gear it will be. So the OP is incorrect in how he is doing his math, he is basing everything off of a single rng roll, which is incorrect.