Hi,
I think the logic behind this as below,
These two fields are there for reference to the store person who is posting GR, just to ensure that whether the delivery is direct to different vendor. The store person can't go to each PO from GR just to ensure this, that's why SAP provided this information in GR itself.
If there is no third party or sub-contracting direct delivery from vendor then system updates the PO vendor in the receiving vendor field. If there is direct delivery from vendor to sub-contractor then system updates 'Sub-contracting' vendor code (which is defined in the PO under delivery address tab 'SC' vendor) in receiving vendor field.
Also, if there is a goods supplier in the PO vendor partner function then this field becomes 'Supplying Vendor' and the goods supplier vendor from PO is displayed in this field.
So, this field information is for reference to the store user.