If a Break Total row is added to the Pivot by using the "Add Totals" menu and surface values are enabled, the following Pivot display results:
With surface values enabled, the various Break Total cell values are evaluated using the displayed values of the "Units Sold" Fact column. For example, the "ColMax Units Sold" total line value has changed (from 500) to 1,100 since the maximum value displayed in the "Units Sold" column is 1,100. The "Average Units Sold" total line value has changed (from 210) to 525 since the displayed sum of "Units Sold" is 2,100 and the displayed number of Cities is 4 - leading to an average calculation of 2,100/4 = 525.
In either case, whether surface values are enabled or disabled, the computed item Break Total values are calculated by applying the computed item calculation on the Break Total value for the column on which the computed item calculation is based.
There exists no facility whereby computed item Break Totals are evaluated by summing (or some other aggregate data function) its constituent displayed Detail cell values. If such a facility were to exist, the "Total" cell for the "ColMax Units Sold" column above would be 4,400 (= 1,100 + 1,100 + 1,100 + 1,100) i.e. the sum of all Detail Fact values in this column.