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>>Are you SUMming the variable twice? In other words, if you create a report variable and set the calculate option to SUM ... and then when you use the report variable in the Data Group Footer you also mark that as SUM .. you have now told it to SUM and already SUMmed variable which will give you way more than you want.
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>No, I am not SUMing the value twice. I have a variable nTotAmt which is set to SUM() field/expression PO_AMOUNT (name of the field). Then in the Group Footer I display this variable. The variable correctly SUMs the value but it does it for every record, both the PO Header and PO Item.

How about qualifying it with an alias?