"Strong leveling effect" in this context means that if Excise Tax were collected on all excise goods (regardless of whether they were stockpiled or not), it could lead to a situation where prices of all goods would become equal.

To avoid this, the UAE government introduced a provision in the Excise Tax legislation to collect taxes only on the excise goods that are actually sold or consumed, rather than on all goods. This way, there would be no market disruption caused by a sudden increase in taxes on all goods, which could have led to the same price for both newly produced and older stockpiled goods.