In the material master (MRP 1 view) for the material, you have set the indicator for DY for Dynamic lot size creation in the Lot size field and the Lot size independent costs and storage costs indicator.

Starting from the shortage date, successive requirements are grouped together to form lots until additional storage costs become greater than lot size independent costs.

The most appropriate lot size is 3000 pieces because an additional requirement of 1000 pieces for the 27 July would mean that the lot size independent costs would be exceeded.