Invoicing data at order line item

Dear Gurus,

I am crating a export order and entering two line items.

when I go to individual line item i seen that at billing document tab page of line item " Invoicing data" field is blank bu on the second line item invoicing data calling a value.

I am just confuse, that if both material are for selling and almost same characteristics, than why here is two different behavior, from where this value is coming on the line item.

thanks. looking for the gurus feedback.

regards,