Need to understand the logic behind calculation of Scheduled Ship Date

Summary

Need to understand the logic behind calculation of Scheduled Ship Date

Content

Hello,

We created a Sales Order on 20-MAY-20 with a Requested Ship Date of 27-MAY-20 (the item does not have onhand).
Created another Sales order on 20-MAY-20 for the same item with a Requested Ship Date of 25-MAY-20. 
As there is no onhand, GOP seems to consider the Item Lead Times (Pre, Processing and Post lead times) to derive the scheduled ship date.
However, in both the above orders the scheduled ship date is the same date (regardless of the difference in Requested Ship Date). 
On analyzing further, we observed that the item lead times start getting added from the last run date of the Refresh Order Promising Server.(20-MAY-20 in this case)