Merged lines for multiple units including tax lines as single units in total count

Hello!  We have an invoice for 50 units.  The tax lines on the invoice, as default Oracle behavior, go into the Mass Add table separately from the item lines.  There are four different tax rates associated with the receiving location, so there are four separate tax lines.  When the item lines and the tax lines are merged then split, the sum split is 54 units, not 50.  How can we get this to split correctly with doing a manual import?