Not only have you added new rows, you've deleted one that was there previously. Moving targets are rather hard to code for. Try:
Code:
Sub MailMergeToDoc()
Application.ScreenUpdating = False
Dim Tbl As Table, r As Long, c As Long
ActiveDocument.MailMerge.Execute
For Each Tbl In ActiveDocument.Tables
With Tbl
If InStr(Split(.Cell(1, 1).Range.Text, vbCr)(0), "Total Reward Statement") > 0 Then
For r = .Rows.Count To 1 Step -1
If Split(.Cell(r, 1).Range.Text, vbCr)(0) = "Shares Total Reward Statement" Then Exit For
Select Case Trim(Split(.Cell(r, 1).Range.Text, vbCr)(0))
Case "PSP Award", "DBP Award", "RSP Award"
If Split(.Cell(r, 4).Range.Text, vbCr)(0) = "" Then
For c = 4 To 1 Step -1
.Cell(r, c).Delete
Next
End If
End Select
Next
End If
End With
Next
Application.ScreenUpdating = False
End Sub