I'd like to think that it is also affected by your Grimoire score, not in the sense that the higher the better, but that your RNG changes with your score. To me, that explains why long time players experience the same drops over, and over (little to no score change). While, newer players seem to have all the luck (more chance at score change). I just got the vex twice in two weeks on my first and second HM completions. My Grimoire is just over 2k atm.

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.

yeah, i know it was an example. and i followed up on your example :) but if it is harder to get the sought upon weapons, why do kinderguardians get them so easily?

I don't know. It is frustrating though, ain't it. I think that some people just are really lucky, after all, if it is random (even if it is weighted) there are millions of players, some of them are bound to get multiple lucky rolls, and they are the ones that will brag about their good luck on here. I have about 500 hours in, and, the only exotic weapons I have gotten as drops are; Mida, Hard Light, P&T (twice, after I already bought it from Xur) and Ice Breaker (again, twice, after I already bought it from Xur).