Planning worksheets question

I have 2 sales order of Item SPK-100, begin inventory is 0.

SO 1 :

Order date 28/01/10, Item SPK-100, Qty : 30. And create Qty ship with posting date 28/01/10 : 10.

Item availability by periods

27/01/10 : PAB = -10

28/01/10 : GR = 20, PAB = -30

SO1

Order date 29/01/10, Item SPK-100, Qty : 30.

And I run Planning worksheet. It have 2 action message of SPK-100 which have qty is 20, 30 at 27/01/10 and 28/01/20.

I think the correct is 30, 30 at 27/01/10 and 28/01/10. So how Navision calc planning quantity?

Thanks for your help.

Sorry for my English.