Why does MERGE DELTA OF produce an UPDATE DML statement against the table being delta-merged?

For Sarbanes Oxley auditing I audit DML statements to SAP-owned data by users other than SAPSID. This a monthly report of this activity is almost always empty but recently I discovered that the SQL Statement "MERGE DELTA OF...." executed by user system against a SAP-Owned table also produces an "UPDATE" record in the audit log associated with that Delta Merge operation.

As advertised, the only syntax that the UPDATE dml statement allows is 'SET COL_NAME=<some

Does anyone know what is being "UPDATED" ?