When MRP generates a top level job, it is natural to expect to see associated lower level jobs and POs generated during the same MRP session. This would be the case if all manufacturing events were to occur on the same day.
In reality, however, manufacturing events unfold over time. Any subassembly item that is required immediately will be given a job within the current MRP session. But any subassembly item that is not required at this time will be given a job in a future MRP session. That future job quantity may incorporate additional demand for the item that happens to materialize in the intervening time.
The same principle applies to purchased items. Any item that is required immediately will be given a PO within the current MRP session. But any item that is not required at this time will be given a PO in a future MRP session. That future PO quantity may incorporate additional demand for the item that happens to materialize in the intervening time.
What determines when an item is required? An item is only needed now when its required date falls within its planning period, which is the number of days allocated by MRP for making or buying the item.