Process order confirmation and reversal showing different amount

Hi Sap Expertise,

I have came across to such scenario for Process Order Confirmation and reversal.

1. Order Confirmation - Mov. Type 101

Inventory  --- Dr 23,189.15

COGM ------- Cr 23,189.15

2. Order Confirmation reversal - Mov. Type 102

Inventory  --- Cr 23,189.15

COGM ------- Dr 40,758.42

Prod. Var.--- Cr 17,569.27

I want to know in which sense SAP is calculating Prod Variance?

What are the possible reasons to clarify whether  posted amounts are right or wrong.

Thanks in Advance....

Regards

Sagar