I think this. There's one guy I play with and we get completely opposite exotic drops. E.g. He's had Thunderlord, Mida, Vex and I've got Gally, icebreaker. I'm also convinced it's weighted so the first times you do E.g. A raid you're more likely to get an exotic. This then decreases, so if you're loot table has a low chance of Gally, it then drops to practically nothing. And Xur isn't random. Sells Last Word before Trials. Sold Red Death just before the pulse buff. Truth before Trials. No Land Beyond when loads of YouTube videos of it started coming out. It might have been random once, but in the run up to HoW it was as if they were giving all the good stuff that was previously rare so people could complete their collection.