How safety stock will work in Ax 2009

Dear All,
Case I: working fine.
If I want to produce the 1000 kg of Finished item (FG-001), required 187 kg of RM-001.
FG-001 coverage group is “Requirement”
RM-001 coverage group is “Period”
I created the sales orders
SO-001 – FG-001 – 15000 kgs delivery receipt date is 16-10-2013.
SO-002 – FG-001 – 1000 kgs delivery receipt date is 16-10-2013.
SO-003 – FG-001 – 3000 kgs delivery receipt date is 16-10-2013.
Posted the safety stock level for the item RM-001 with 5000 kg
When I run the master schedule for the item “FG-001” system generates a planned purchase order for “8553”.
Case II: Not working fine
Everything is same, but I changed the delivery receipt date of each order.
SO-001 – FG-001 – 15000 kgs delivery receipt date is 30-10-2013.
SO-002 – FG-001 – 1000 kgs delivery receipt date is 30-10-2013.
SO-003 – FG-001 – 3000 kgs delivery receipt date is 30-10-2013.
When I run the master scheduling for the item “FG-001” system generates a planned purchase order only for “7805”.

Why system is not suggesting “8553”. Is there any problem in this case.

Thanks & Regards,
Nanjappa