Your maths is off, but I get what you are saying. Each time there is a 1 in 6 chance of getting an item. Next time there is a 1 in 6 chance of getting the same item again. This is the same whether you roll once or 100 times. It is 16.3% every single time. There's no cumulative effect. However consider this - there is a 5 in 6 chance you will get a different number, or item in this case, everytime the dice is cast so to speak. Thats an 83.3% chance something else will come up with every roll of the RNG - every single week. I can only think that they choose the items or certain items are more likely to be rolled and it's not a straight 1 in 6 chance for every item.