If you're talking about drops they are RNG. Xur is random but the random draw happens in advance of his arrival and apparently now there are two Xurs they can pick from according to Megamanexe. The reason why you see people get multiples of items is because given a large enough sample size even unlikely events will occur, like someone getting 5+ gjallarhorns or 7 Suros Regimes. Each player's experience is a very, very small sample of a very large data set.