Code is appending when it should be adding (SUM)

Question

I altered this code provided by @Grove, so that instead of adding columns "G" and "H" it adds "F" and G"

Sub merge_rows()
        Dim last_row As Long
        Dim row As Long
        Dim s As Worksheet
        Dim col As Integer
        
        Set s = ActiveSheet 'use this line to process the active sheet
        'set s = thisworkbook.Worksheets("Sheet1") ' use this line to process a specific sheet
        
        last_row = s.Cells(s.Rows.Count, 1).End(xlUp).row 'find the last row with data
        
        For row = last_row To 3 Step -1
            If s.Cells(row, "D").Value = s.Cells(row - 1, "D").Value Then
                ' found a match in column d
                
                ' add column G
                s.Cells(row - 1, "G").Value = s.Cells(row - 1, "G").Value + s.Cells(row, "G").Value
                
                ' add column H
                s.Cells(row - 1, "H").Value = s.Cells(row - 1, "H").Value + s.Cells(row, "H").Value
                
                'append all other columns if different
                For col = 1 To 6
                    If Not s.Cells(row, col).Value = s.Cells(row - 1, col).Value Then
                      s.Cells(row - 1, col).Value = s.Cells(row - 1, col).Value & " " & s.Cells(row, col).Value
                    End If
                Next
                
                ' now delete the duplicate row
                s.Rows(row).Delete
            End If
        Next
    
    End Sub

I changed the lines to:

' add column F
                    s.Cells(row - 1, "F").Value = s.Cells(row - 1, "F").Value + s.Cells(row, "F").Value
                    
 ' add column G
                    s.Cells(row - 1, "G").Value = s.Cells(row - 1, "G).Value + s.Cells(row, "G").Value

Now, Instead of adding column F, its appending it.

I can't see how the change I made would have any affect on it being added, any insight would be appreciated.