The probability of getting 5 chest pieces in a row is ridiculously low though... .032%. RNG isn't truly random in these games though, considering it's based on a computer running an equation/formula.
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it's not true RNG and it's most likely weighted. true RNG would be if a boss could drop 3 things, they would all be 33% but it's probably more like one is 60%, one is 30% and one is 10%.