Compare the above Pivot section display with the following example. In this example, surface values have been enabled.
As you can see, the "ColMax Units Sold" and "Average Units Sold" Fact columns values have changed. With surface values enabled, the ColMax function refers to the displayed "Units Sold" Fact column for input data and the highest value in this column is 1,100 which is the value that is displayed in the "ColMax Units Sold" column above.
The Avg Computed Item function which is used to evaluate the "Average Units Sold" column takes its input from the sum of the displayed "Units Sold" Fact column values and divides this by the number of displayed occurrences of values in the "Units Sold" column - this is 2100/4 which equals 525 - the value displayed in the "Average Units Sold" column.
In general when you enable surface values for Pivot section computed items, they operate on the summed, aggregated values from the underlying Table section.