abnormal GR/IR posting logic for MR8M.

I have a question for GR/IR posting logic for MR8M.

If there are no GR but IR (or already cleared GR & IR).

like this.
===================
IR 1 100qty 100USD
IR 2 100qty 100USD
IR 3 100qty 100USD

total WRX = 300 USD
===================

If we cancel IR 3,
I thought - WRX amount would be 100 USD (* reffered by sap note 46564)

by calculating ( 100qty(reversal qty) / 300qty(total qty) )* 300 USD(total WRX).

But the system have posted 300 USD for WRX by canceling IR 3.

And there were 200 PRD ( 300 WRX - 100 IR = 200 PRD).

I am a bit confused.

What is the correct posting logic ?

any reply would be appreciated.

thank you in advance.