Well, it's probably more because I'm bad at explaining things. Especially probability. I'm sure it can be calculated much more exactly than I have. Your point is still somewhat valid as the odds of getting any item repeated several times in a row is still small but it could just be bad luck or uneven odds. It could also be Bungie picking everything manually but it doesn't seem to make sense to me why they'd repeat items so much as it annoys everyone. They could be trying to stretch out the grind for Exotic items but I doubt it.
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Well i'd assume because most other exotics help during PvE content while the Voidfang are Pvp based, that being said look at things that have been sold recently truth, patience and time, the last word, mida multi tool, mainly pvp based, because of the fact they want to extend game life Pve content must be hard to unlock, whereas Pvp content can never be finished allowing end game items not ruining the excitement of playing