If your expression includes a ratio, or some other formula that is handled differently in the total, such as distinct or fabs(), then the expression total will not be the same as sum of rows. For example, in a ratio, sum of ratios (sum of rows total) is not equal to the ratio of the sums (expression total).
If this a straight table, then select the sum of rows option on the Properties | PresentationExpression tab.
If this is a pivot, you will need an expression of the form:
=Sum(Aggr(<your expression>, <table dim1>, <table dim2>, ...))
If you upload your qvw, or sample, then you will get more specific help.
Please find attached file for solution.
TimeStamp(Timestamp#(Hour, 'hh:mm:ss'), 'hh:mm:ss') AS Time_Formatted
Expression in Straight table:
=If(RowNo() = 0, Interval(Num(Sum(Time_Formatted)), 'hh:mm:ss'), Sum(Time_Formatted))
test.qvw 146.2 K