SUM two different tables columns in SQL ACCESS

Trying to figure out how to get this JOIN to work properly. Been sitting here for about 30 minutes. Can someone help me out? I am trying to subtract one form the other, to see the difference between invoice quantity and inventory volume.

SELECT Invoice.NameOfItem, SUM(Inventory.Volume - Invoice.Quantity) As TotalNeeded
FROM Invoice
INNER JOIN Inventory
ON Invoice.NameOfItem=Inventory.NameOfItem
GROUP BY Invoice.NameOfItem;

The issue is the output is incorrect.

 SELECT NameOfItem, SUM(Quantity) AS TotalNumberNeeded From Invoice GROUP BY NameOfItem

subtracted from

 SELECT NameOfItem, SUM(Volume) AS TotalNumberNeeded From Inventory GROUP BY NameOfItem

Is = -112. The output is currently "992"