Partial cancellation of billing items

Hi, can anyone advise the following? I created a billing with 2 line items. If i have not release billing doc to accounting, system does not allow me to select and cancel item 1 when i perform VF11 (Invoice cancellation). However, it's possible to cancel item 1 using selection list in VF11 if billing already released to accounting. May i know why the inconsistency in treatment?

Thanks

PP