It's not RNG and his math is correct by the way. You can't look the statistics individually by the way. You have to remember the chances for the same to item to appear 4 times in a row with the other items have the same chance.
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I never said his math was incorrect, just that the fact that the chance of 4 voidfang being sold in a row being low is insignificant, and does not prove or even imply a lack of randomness in the pattern.
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It kind of does. 4 Voidfangs appearing in a row has 0.077% chance of happening if it was RNG. Those numbers are pretty low.