Tableau automatically aggregates measures on the view canvas. Default aggregation in Sum.
When you drop a measure field, the aggregation applied is Sum.
This aggregation can be changed to Average or any other type. When an “aggregated measure” is used in a calculation and then the calculation is placed on the view canvas, it shows Agg in front of the calculated column. It means that aggregation is built within the calculation. This aggregation cannot be changed.