When and how does Qty Available field in Inventory get update from Purchase Orders

I would really appreciate if someone could clarify this for me:

From testing it appears that the Quantity Available field in the Inventory Maintenance and other screens get updated as follows:

When PO is raised - The Qty on PO field is updated , say with 400 items, but Qty Available not 
When the first receipt is done on the PO- The Qty Available gets update by the full Qty on PO , even if only one item is received.
Is this as designed and if so what is the logic behind that?

Thank you in anticipation!