I don't know. It is frustrating though, ain't it. I think that some people just are really lucky, after all, if it is random (even if it is weighted) there are millions of players, some of them are bound to get multiple lucky rolls, and they are the ones that will brag about their good luck on here. I have about 500 hours in, and, the only exotic weapons I have gotten as drops are; Mida, Hard Light, P&T (twice, after I already bought it from Xur) and Ice Breaker (again, twice, after I already bought it from Xur).