Hi Eugenia
You are using MRP type PD, which means that MRP will create the purchase requisitions to cover specific requirements. Therefore, the date will generally reflect the requirement date. Also, you should notice that the first purchase requisition is already firmed, therefore, it was is not changed by MRP anymore.
The following link from SAP on-line help should be useful for a better understanding of the dates calculation carried out by MRP:
Backward Scheduling for External Procurement - Material Requirements Planning (PP-MRP) - SAP Library
BR
Caetano