It's just RNG loot (random number generated) statistics. Most people will get them all within 14 runs or so, and then outliers like you and me will get the majority of them more than once but have trouble getting 1 or 2. For me it's the hunger. And until recently the light of the abyss. Same concept for exotics. I've gotten 7+ NLB's, 4+ Suros regime, 3 Gjallerhorn's. No hawkmoon :( or fourth horseman. And I know people that have gotten 4+ hawkmoon but are still waiting for their Gjallerhorn. A lot of people don't like the RNG system as they have trouble getting the guns they want/need and are sick of getting killed by said guns in the crucible or such. But I personally like the RNG loot system, although it could use some tweaking, as it gives me something to play for, and ensures a random level of rarity in the game.