Maths is correct, but only to say to get the same armor for 4 consecutive weeks is a probability of 0.077%,
However, each week is an independent event. so the probability of a particular armor piece coming up this week is still 1 in 6.
For Voidfang to come again next week, the probability will be 1 in 6 as well.
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I was only speaking of the chances of getting it 4 weeks in a row.
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But from Maths you concluded that Xur is not RNG. I think Liam hit the nail on the head with his analogy.