You do not understand the value posted to the GR/IR Clearing Account (WRX line in the Accouting Document) in a return movement (122) or in a cancellation (102).
The valuation of reversals (movement type 102) and return deliveries (movement type 122) is based on the previous postings in the PO History.
For the valuation of the reversals the system compares the quantity received so far against the quantity invoiced so far.
1) When the invoiced quantity is higher or equals the received quantity, the reversal is valuated according to the invoiced value:
Reversal = Quantity in the cancellation x (Invoiced value / Invoiced quantity).
2) Otherwise, the valuation is based on the clearing value:
Reversal = Quantity in the cancellation x (Clearing Value of the Invoices - Goods Receipt Value) / (Invoiced Quantity - Goods Receipt Quantity).
Note: The Clearing Value of the Invoices might be different from the Invoiced Value. The Clearing Value of an Invoice is the amount posted the GR/IR Clearing Account in the corresponding accounting document (WRX posting key). It can be seen in the Purchase Order history switching to the Clearing Value view or directly in the table EKBE (field AREWR for the local currency and field AREWB for the PO currency).
If no invoices have been posted, this formula is reduced to:
Reversal = Quantity in the cancellation x (Goods Receipt Value) / (Goods Receipt Quantity).
So, the reversal is valuated according to the average values received as explained in the note 49995 Case A.
Related notes / content:
KBA 1827734 - Valuation of Goods Receipt Reversals and Cancellations
SAP Note 49995 : MMIM: Valuation of return deliveries and GR revrsls
SAP Note 212286 : Overview note: Valuation during goods movements