Assuming the chances of a certain piece of armour dropping is a straight 1 in 6 chance, and the game simply pulls from those items every week... Then every week it is a one in 6 chance. The math works the exact same as flipping a coin. The chance of getting heads is 50/50 every time you flip the coin, regardless of what the result was on the last flip. So one item appearing one week will still have the exact same odds for appearing next week. The math resets each week.