You misunderstood what he was saying. He said the probability it occurs 4 consecutive weeks. So it is dependent on the previous weeks outcome. Edit: I misspoke, he said the probability that when chest armor goes on sale, it is voidfangs, four occurrences in a row. That doesn't change anything but I thought I should clarify because I think that is where the misunderstanding came. Source: I've taken enough statistic courses in college to make most lose their minds