I don't think it's necessarily predetermined, but I do think that the system is weighted toward dropping items that have already dropped for you... Been playing since day one, and I've gotten Truth 10 times now... 4 times in the last month.. Had never gotten Gjallarhorn up until Wednesday of last week... Two more have dropped since then... Got my second one on the Deathsinger the same day that the first dropped, and the third from the exotic chest in the VOG.