Previous Approved Amount- Invoice

Hi,
Version: 6.3.1

while approving invoice manually, what is the significance of column:"Previous Approved Amount"? user tried creating invoice manually with 3 line items and approved it , thus voucher is created. Later realized that amount entered while approving first time(say 100 USD) was incorrect for one of the LI and hence tried to approve it with different amount(say 110 USD) for that LI only. In this case, the "previous approved amount" showed the last time value being approved. second time approving created another voucher. However when again user tries to approve the same LI with another different amount-say 105 USD(this time correct as user accidentally entered wrong twice), the "previous approved amount" showed the amount approved first time(100 USD) and not 110 USD. Is this an OTM bug or standard functionality(if yes, what is the business reason). Please let me know.

I cannot unapprove invoice every time(to remove the voucher) as the invoice has many line items and user does not want to go with that process. neither create another different invoice as it is more data entry issue.

Also figured out that this "previous approved amount" display with amount feature works(while approving invoice second time) only when there is matching Shipment cost line.

Thanks.