Mismatch in Account Modif in PO Account Assignment Category

Hi,

I am raising a PO with 4 items and Account Assignment Category as "K" (Cost Center Category). In OME9, I checked the account modification mentioned is VBR. In Material Master I checked the Valuation Class as 3219 and 3010.

I wanted to change the GL in PO, hence referring to the above information I tried changing the GL in OBYC. So I changed the GL in Transaction Key GBB- VBR against the Valuation Class 3010 and 3219. When I checked the PO creation with the same materials, one of the Line Item GL account remains unchanged. When I go back and check in OBYC and GBB, I see that the material is picking the GL account from Account Modification INV.

Hence I changed the GL in GBB-INV, the GL is correctly picking in PO.

I want to know, when my Account Assignment Category in OME9 is containing the Account Modification as VBR for PO, then why the GL in PO is picking from Account Modification INV in OBYC.

Kindly explain.