it's because you have introduced another table which has multiple records with the same loadnumber
try
Code:
SELECT [Load Number], Sum([Quantity Ordered]) AS [SumOfQuantity Ordered]
FROM completed
GROUP BY [Load Number]
and if you want to pick up load numbers without any quantities ordered
Code:
SELECT DISTINCT P.[Load Number], Q.[SumOfQuantity Ordered]
FROM [Pad Wk1] P
LEFT JOIN
(SELECT [Load Number], Sum([Quantity Ordered]) AS [SumOfQuantity Ordered]
FROM completed
GROUP BY [Load Number]) Q
ON P.[Load Number]=Q.[Load Number]