Add values from one column based on values in another column

I am trying to add the values in column A for all rows that have the same value in column B. My code will sum but the values are incorrect, off by a value of 2 to 3. See attached spreadsheet.

Option Explicit

Sub calc()
    Dim LastRow As Integer
    Dim sum As Integer
    Dim x As Integer
    Dim i As Integer
    
    LastRow = ActiveSheet.UsedRange.Rows.Count
    sum = 0
    x = 2
    
    For i = 2 To LastRow
        If Cells(i, "B") = Cells(i + 1, "B") Then
                sum = sum + Cells(i, "A").Value
        Else
            Cells(x, "D").Value = sum + Cells(i, "A").Value
            sum = 0
            x = x + 1
        End If
    Next i

End Sub