SO getting completed without full delivery

Hi,

We are creating deliveries based on schedule lines in the SO. All the schedule lines with the same date will create 1 order and so on.

For some of the SO we have noticed that the number of deliveries created are not as expected. Say, when expecting 3 deliveries we only find 2 deliveries (which are correct) and still the status of the SO changes to 'Completed'

I need to know why without creating complete deliveries is SO being completed