Vendor confirmation quantity is reduced in IBD ( background processing )

Vendor confirmation quantity is reduced against PO item quantity , this is single background processing program which creates both PO and IBD in single run ( BAPI ) . PO line quantity is 4 CA but IBD (LA) vendor confirmation quantity is 2CA . What is the reason for vendor confirmation quantity reduction ? , any error analysis note for this ? or any relation with MRP relevant check ?.