This does not prove that xur is not rng. Each time xur picks armour there is a 1/6 chance of any piece of armour being chosen. Random chance does not remember the previous result. Sometimes random chance screws you over. You know this if you've ever played a tabletop rpg and rolled all 1s. On a side note I think xur should be rng but make him choose something he has not picked until everything has been picked. After everything has been picked start the cycle again.