You do realize that probability doesnt change like that. For example if you flip a coin 3 times and all three times it is heads, when you flip the coin a forth time, the chance that it will be heads is still 50%. This means that if he works by a system where it first chooses head, arms or chest, and then the armour piece, there is a 1/3 chance that it will be chest, and then out of that it is again 1/3 chances (i think there are three chest pieces), that it will be voidfang. This probability does not change per week, unless it is put into the programming that if this item has been sold before that now it has a lower chance of appearing (which doesnt seem to be the case). There for every week the chance of it being voidfang is the same, no matter what happened last week. Voidfang just so happens to be what has appeared for these past few weeks. People always take into account the past when looking at probabilities, but we forget that machines do not, and so our calculations end up being off.