why is invoice not released automatically?

Dear friends.

Here is  the scenario.

1, PO, quantity=10

2, GR, quantity=10

3, LIV, quantity=11 [ surplus 1 ]

at this point, the invoice is blocked since one more piece are taken into account..

and at this time , the quantity in PO are totally received. It's not possible to  make any further  GR.

but  changing  the quantity in PO

4, Change PO,  quantity=11

5, GR, quantity=1

Now , both the  GR quantity and the IV quantity are 11 eventually.

and nothing is wrong with the Balance.

so, the invoice is supposed to be released now, but it is still blocked  showing quantity has difference  1 piece.

why does this happen?

Even more, when I using MR11 to clear the GR/IR account, the difference will be treated and posted .

How to deal with it , I think the FI account is totally wrong,

can friend offer some information?

Best regards

Tang