So, Xur told me something very mind blowing last time he was here, when I bought an exotic form him. He said: "I am just the trash collector of The Nine." DO YOU KNOW WHAT THIS IMPLIES!? Those exotics, the ones he gets from The Nine,are TRASH. The ones they've thrown away, either because they had no use for them or were imperfect. But, you may ask, what about all the guns that have back stories to them? Well, it COULD be possible that The Nine are replicating these guns, for reasons unknown. Or, building on the theory that the Destiny universe is actually reset, or progresses forward in time until the universe 'resets', every time you die/spawn/start a raid or strike. They're pulling those guns from universes where, most likely, they are no longer needed. Why throw them out, if this is true? They might be looking for something. Who knows. Not very well thought out, but I think I'm onto something here.