I have a theory similar to yours, but I think once you get a certain weapon to drop it increases your odds of getting that weapon (thus making it less likely to get the ones you don't have). For example, I didn't get Thunderlord until a month after CE came out and I've been playing Destiny almost exclusively since day 1 (can't stand there being stuff that I don't have lol). But since I got Thunderlord it has dropped 3 additional times. Same thing happened with Universal Remote, I've gotten 5 since my first one dropped last month. I currently cannot Hunger of Crota or the chest piece for the hunter from CE and I have ran normal first on all 3 of characters since the dlc dropped. I've gotten every other raid weapon multiple times and the crux twice. I have also never gotten Hardlight (I am Xbox so PS4 exclusives are not yet available). Whatever the system is that they are using, it is designed for us to never get that one last thing (or 2, or 3) so we keep playing.