It may seem like it is weird, but on l2off items are in groups and these groups have various chances. So when you kill a mob, first the chance of the group is evaluated. When positive, chances within the group are evaluated and 1 item inside the group is selected and dropped. So if the group chance can be multiplied without overflowing , it is multiplied. If not, then either drop amount is increased or group is replicated, or combination of the 3 so that the best balanced scenario of 16x is achieved.