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Maybe TJ can correct me here, but the custom column is only going to report on people who match Both the Report Smart List constraints AND the custom column smart list, so you will see Everyone who matches in the Report and a second column of people who matched Column 1 and Column 2.
eg:
Col 1: Leads by Month Report with Report Smart List = "Lead Status = Open" Col 2: Leads with Lead Status= Open + Smart List 2: Lead Source = Marketo