Xur is RNG. It is possible all items are not weighted similarly so they are harder to get. For example: Let's say that the RNG can only generate numbers 0-100. So let's say it generates a 45 this week. Then that number is compared against the programed criteria. 0-75 void fang 76-95 chest b 96-100 praxic fire. Still an RNG, just not equal weights. There is no way in hell bungie is paying someone a salary to pick what xur is selling