Its absolutely random for everyone, no one ever know if or when they'll get a dungeon or raid exotic. Thats the point...its random. Especially since they're meant to be the rarest loot in the game. Yes some people get lucky, but me personally I've ONLY ever gotten lucky once by getting vew on my 2nd run. Thats it...literally never been lucky like that ever between D1 and D2 besides vex.