I will use the Superstore data set for this example.
For example, you want to get the sum of the average sales for each customer segment in the Grand Total row. But what you are getting now is the overall average of the data source, which is as follows:
How is Tableau calculating the Grand Total? It is performing the same calculation for the measure as it does for the Dimension in the view, but at a higher level. In this case, the entire data source is considered instead of each customer segment. Tableau uses all the dimensions on the columns to calculate column Grand Total. Same goes for row Grand Total.
In the example, ...