Schedule line and manual purchase order priority issue in MD04

Hi Experts,

I have one question in MD04:

After MRP run, there is SL generated which has delivery date 15th 09 2016, after that we created one purchase order manually which has delivery date 6th 10 2016. However in MD04 system will pull in the manual purchase order instead of schedule line.

I wonder if there is something wrong with the result.

Appreciated if someone could help explain system logic why it's like this. Thanks a lot in advance.

I have attached MD04 screenshot as below for your reference.