What is diff between Still to be invoiced for Asset Po & Service PO?

Dear Experts,

I have gone thru all the Posts & notes but unable to identify how still to be invoiced worked for Asset PO & Service Po in Report ME2N..PLease check below screenshot.

First line item is Open PO for asset. I can understand that GR is not posted hence it is showing values in To be delivered & so values are coming in still to be invoiced. But in second line item which is also an Open Service PO where values are coming in To be delivered column but not showing in still to be invoiced. And if i posted an Service entry then it shows value in Still to be invoiced.

Both Pos are ticked for Gr based invoice verf. Please help me to understand this behaviour.

Regards,