The Suros/Ghorn thing was just an example, I have no idea what Bungie has actually done, but it seems obvious that the RNG is weighted to make the most popular weapons the hardest to get. It makes sense, to me, that once you get a couple of the most sought after weapons, Bungie would want the remaining choice weapons to be even harder to get. I might be wrong, but I think, as you indicated in your original post, that a significant component of the RNG system is to keep us grinding.