Master planning does not consider the ordered quantity although the "Include inventory transactions" is chekced in master plan

Hi,

please consider the following scenario: 

there is a requirement of item X 3 qty 
- run master plan for item X

- the  master plan generate a planned PO  with 3 qty 

- firm the planned PO 

-  when I run the master plan again it is generate a new planned PO again with 3 qty without any consideration to the ordered quantity of the first generated firmed planned PO ?