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