Dear,
Allocation structure is having one or more settlement assignments.An assignments shows that which costs are to be settled to which receiver type.I mean to say here is you can have cost element group for materials , services and labor(internal activity cost) etc..all these cost element groups are assigned in SPRO setting in respective node.
In your case the G/L(All P&L gls need s to be treated as cost elements) which you assigned in your network activities is not mapped in KAH2 master data.That`s the reason second time after assigning it in KAH2 system processed.
Kindly read the SAP help document for better understanding of settlement process using various components in standard SAP.
Settlement Parameters - Settlement (CO) - SAP Library
Regards,
pardhu