Why is the assessable value displayed, different from the amount manually entered in the excise invoice? (Excise for Manufacturer)

Excise duty is displayed based on the assessable value which is calculated using the formula, (actual quantity * rate). When the total amount of the item is entered manually, the amount entered is taken as overridden value. Hence, it is mandatory to enter the assessable value of excise manually by enabling the option Allow Modification of Tax Details for Excise from F12: Configuration screen of the excise invoice.