Hi,
This is as per standard SAP functionality.
When you are performing a partial clarification on the 1st item, system updates table DFKKZP with the set of selection criteria used for clarifying the item against the payment lot and item no.
However, when you are performing the same activity again, system can't update DFKKZP as it has already been updated previously against the payment lot and item no, so then it inserts the record in table DFKKZS.
Hope it helps...
Thanks,
Amlan