Order Level Adjustment producing multiple lines

I have a single order level adjustment used to calculate tax on an item, based on it's volume.  There are multiple possible values of the order reprice group from the item branch within this adjustment.  When I enter an order with items from more than one order reprice group, I get a tax line on the order for each reprice group, when I only want one line for the tax total of all the order reprice groups.  Is there something I'm doing wrong?