Sum returning wrong value

Can anyone tell me why cell aa in row 75 is returning a value of 11 instead of 12?
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Manfred BertlManagerCommented:
Your document is linked to other external documents. Are they related and needed to your cell AA?
robmarr700Author Commented:
Some of the values have been pulled using Vlookups from other excel documents. These values are needed. Ideally I don't want to paste as values as the values on the external sheets may change.....

in Col W change your formula to


instead of your old formula which have text "1"


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