The suggested quantity is much larger than expected

Two inputs multiply here. The order has to cover the target period and the whole lead time, because stock keeps selling while the order is in transit, so a long lead time raises the suggestion sharply. Separately, a supplier's minimum order quantity acts as a floor: if the calculation asks for 12 units and the supplier's minimum is 50, the suggestion is 50. Check the lead time and minimum on the supplier before assuming the forecast is wrong.