While this sounds like conspiracy theory nonsense, I have to say I've had the same suspicions. My buddy had Black Hammer and a full set of raid armor drop after Killing Crota only three times on Normal. But he hasn't had a whole lot else drop for him. I've had g-horn drop for me twice and had every raid primary after two HM Crota runs. I've had every other Crota weapon drop after just a few runs. What won't drop for me are the raid helmet and Black Hammer, even after 20+ Crota kills, several of which were on hard. Other people get them after every run. It does seem rigged ... And it's maddening.