I think Bungie lets everything in Xur's inventory fall to RNG, with the exception of the weapons. I believe that whenever Xur's inventory "rolls" for the upcoming Friday they will look at the weapon and if it already sold too recently they will change it to something else. I would think they were putting more effort into mixing up the weapon selection since we've seen multiple first-timers in a row. Since armor obviously isn't changing I feel they're just letting the RNG fall where it will.