If we assume that each week Xur has an chance to generate each of the exotics, while your math is technically correct (in the sense that he has a 6^(-4) chance of rolling voidfang 4 times in a row) that does not mean that your conclusion is correct. First of all, any particular string of outcomes has an equal chance of happening. He would have the same chance to get 4 different items each week. Second of all, as mentioned 4 events is not statistically significant. In a random system, even when the global behaviour can be determined, local behaviour can always be crazy. It's the same reason why I can kill Atheon 16 times and only receive a vanity item and ascendant materials. So while it sucks that he's once again selling the same armor, the culprit is once again rng.