We have built the 2 columns side by side for users' easy reference. The data from 'Based on Product' column is calculated based on - Qty x Cost. The data from 'Based on Invoice' column is taken directly from the invoice.
They may be a difference in value for the 2 columns attributed by -
i) Overall discount not taken into account under the 'Based on Product' column (Eg. $5 off entire order. Discount for individual item not accounted.)
ii) Calculation error by suppliers on the invoice