So I was just browsing the new Armory section on Bungie.net and noticed that for Thorn it says that "Xur, Agent of the Nine, sometimes sells this item in the Tower". I owned this game since Beta and I have never seen Xur sell Thorn or any exotic bounty weapons. As far as I was aware, Xur will not sell weapons that you attain from exotic bounties. Is this is a mistake or has Bungie changed RNG for Xur to include exotic bounty weapons.
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Probably because the upgrades were sold. And upon upgrading, the game gave you a "new" gun with higher attack, so the armory thinks Xur sold them.