most likely you have 'duplicate' rows of data usually created by doing a join. like a PO table joined to an Order table that might have multiple rows of data with one PO number.
Note that hidden records are included in a SUm so suppressing them only changes how it looks not the actual total values.
Usually a Running Total is the easiest solution but it depends on your data set and report structure and sorting.
A Running Total can use an 'on change of' or formula to inciate to incldue or exclude specific rows.
Running Totals do not work in headers.