Over 1200+ hrs into game, and still have not gotten a gally, vex myth, or thunder lord. O, I've gotten plenty of repeat exotic bounties, 2 plan c's in one day from ROC missions, 4 pat and time, even gotten several regimes and hard lights, all from ROC's, Nightfalls, even several came from crucible. The drop pattern is really screwed up. If anything ever needed fixed it's the loot drop RNG tables.
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Think about it, if I have 17 of 20 droppable exotics whats more likely to drop one of the 17 I have or one of the 3 I don't. It's just basic empirical probability man.