That's why I thought. It seem it does. You simply get the intersection with the set expression in the outer aggregation function. So you should put it there too, really.
Anyway, the expression sum a list of averages calculated over the combinations of the fields Report Year Week,PruID eQuality and TSG Report. I have no idea what the variable VG_DisregardDate contains, but its name suggests that it overrides selections in date fields. Any selection in EQuality TypeOfReport is reset too for the expression and a date range of the last 12 weeks is selected.