Unlimited over delivery check box is reset automatically when i change the PO quantity.

Hello all,

I have created a PR with 15 Quantity and converted the full 15 quantity into a Purchase order. When PO is created unlimited check box is selected by default. Later in ME22N i change the PO quantity manually into 10. In this case Unlimited check box in the delivery tab is reset automatically. Could you please tell me what could be the reason for reset is this system driven settings or standard SAP functionality?

Thanks in advance.