ERP and APS systems are operational systems and are not designed to optimize the inventory targets that they manage. In effect, ERP systems can only put the right products at the right locations at the right time if supply and demand conditions never change.
In realty, of course, almost everything in the supply chain environment changes all the time. Inventory planners know this. Most planners "optimize” inventory using rules of thumb (for example: “demand has risen by 10%, but sales always drop after the holidays, so let’s reduce inventory by 5% or 6%”).