GR going to wrong GL account for PO Item with Account Assignment Category M

Question

Hi Experts,
I need some help on GR posting for PO Item with M as AAC.
PO1 for material ABC with M as  Account Assignment Category, when I book GR for this Item, the GL document generated as below.
Dr: PRD  (Cost Price Differences)
Cr: WRX (GR/IR)
The material ABC price control is "S" and already have standard cost.
For another material DEF which have ascetically same master data as  ABC and got a similar PO2 with M as Account Assignment Category.
But the GR document as below.
Dr: BSX (Inventory)
Cr: WRX (GR/IR)
How can be one same setting got so different results, is there any data in Material or PO make these differences?
Actually the results for material DEF is what we want, how could I avoid the GR results for material ABC.
Thanks~

Answer 1

Thanks you Mukthar,
I have checked all the information and porcess for both material and did a lot of testing in testing system.
Here is what I found.
Case
Material ABC  SO: 1111   PO: 6666 (ACC:M)
Material DEF  SO:2222    PO: 7777(ACC:M)
All the information in material mast data, SO, PO are the same.
The differences is that:
PO:6666 post GR before Material ABC's standard price release. So the first GR document as below
Dr: PRD  (Cost Price Differences)
Cr: WRX (GR/IR)
This is correct document without material stand cost.
The problem we reverse the fist GR document, release standard cost for Material ABC. Then re-do the GR. but the GL document still book into  PRD instead of BSX.
If I create a new SO:3333 and PO:8888 (with ACC M) for material ABC now.
The GR could be book into BSX.
It seem like once the PO got GR with 0 standard cost, even I relesed a new price for the material and re do the GR, it still take the previous 0 cost to book inventory which make all the amount into PRD.
Quite Strange