It means that Xur rolled a die and got the same number 4 times in a row. That's it. Every week is it's own isolated incident, just as every roll of a die is an isolated incident. There is no outside force acting on the die or on Xur that makes either less likely to roll the same thing as the time before. Every week Xur picks a random exotic for every class. There are 6 to choose from. There's no conspiracy here. It's just bad luck.