If you played attention. The day xur had no weapon. Bungie said they couldn't get him to change which indicates they can control his inventory when ever they want. There was also a day where dataminers had xurs exact inventory and what happen? The next day his inventory was completely different. Bungie blocked data miners after that. Why? Because they don't want you to know XUR isn't RNG a long with other secrets.
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Well they most have some control, but I think they leave most of it up to rng