How Quantity flows to MIRO

Hi,

I have a question for the GR based Invoice verification briefed as follows

1. Created GR against PO

101 Mvt ty-- 10 Qty

2. Posted return delivery against 101 Material Doc

122 Mvt ty-5 Qty

3. Now If I go to post MIRO, I can see the quantity proposed for invoicing 5 Qty, Why?

Please help me to understand what is the logic behind and how the Calculation takes place for the quantity field in MIRO, Is there any note available ?

Thanks