The problem isn't that your math is wrong, so much as that you're misinterpreting it. The chance of getting the Vestments four weeks in a row is indeed (1/6)^4 = 0.08%, but that's also the chance of getting [i]any[/i] specific arrangement of four Warlock items over the same span. That is, there are 6^4 = 1,296 different permutations of items Xur could have offered, and one of them had to have been selected. This one sticks out not due to mathematics but rather due to cognitive bias. With that said, though, it's absolutely time for him to offer something else for us. As long as that something else isn't Sunbreakers.