Hello,
please, read this instructive thread:
http://scn.sap.com/thread/3479677
Anyway, besides the fields defined in splitting criteria, all the fields written in header data of the invoice (so in table VBRK) are automatically splitting criteria. So check if any field stores the Purchase Order number of the customer (i.e. VBRK-BSTNK_VF).
Best regards,
Andrea