Multiple schedule lines in Sales order.

Dear Gurus,

When ever i am trying to create a sales order with material "A" system is showing two schedule lines in Sales order.

For eg:- I have 10 Qty in Stock for Material "A".

Now let say I have created an order today (29-July-13) for 1 Qty. System is confirming stock on (31-july 13). Even though i have enough stock in hand.

and If i create an order of 11 Qty. Then system is confirming first 10 qty on 31- July- 13 and remaining 1 qty on 6- August-13.

Could anyone tell me how system would be calculation these days. From where it is picked up... I have check in Materail master in MRP tab..no days are maintained.

Regards,

Rikin