EDI Release management Schedule ship date issue

Hi All ,

After processing one EDI shedule (type - Planning/Release , Purpose - replace all ) one So line is updated with schedule ship date -11/10/2017 23:59:00 with qty 2 for item .

User have created manual schedule (type - Planning/Release , Purpose - all) after processing this schedule same sales order line got updated - new qty is 10 and new ssd is 09/11/2017.

My understanding is Same sales order line got updated as qty 2 old schedule qty + qty 8 new schedule qty as start date is same on both the schedules with same item for same customer . We need to know why schedule ship date is changed and how SSD calculations are made,