Why is RecordsAffected=-1

I must be blind because I can't figure out why RecordsAffected=-1 in this trivial example.

Here's my SQL Server 2000 table definition:

if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[tblAmenitySpec]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [dbo].[tblAmenitySpec]
GO

CREATE TABLE [dbo].[tblAmenitySpec] (
      [AmenityID] [int] NOT NULL ,
      [AmenityTypeSpecID] [int] NOT NULL ,
      [Flg] [bit] NOT NULL ,
      [Qty] [decimal](18, 4) NULL ,
      [AmenityAttrValueID] [int] NULL
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[tblAmenitySpec] WITH NOCHECK ADD
      CONSTRAINT [PK_tblAmenitySpec] PRIMARY KEY  CLUSTERED
      (
            [AmenityID],
            [AmenityTypeSpecID]
      )  ON [PRIMARY]
GO

Here's the update stored procedure:

CREATE PROCEDURE dbo.proctblAmenitySpec_UpdateForAmenityIDAmenityTypeSpecID
      (
      @AmenityID INT,
      @AmenityTypeSpecID       INT,
      @Flg  BIT ,
      @Qty  DECIMAL(18,4) ,
      @AmenityAttrValueID      INT
      )
AS
-------------------------------------------------
-- local variables
-------------------------------------------------
DECLARE
      @Proc            VARCHAR(50),
      @RowCount      INT,
      @ErrNbr            INT,
      @ErrMsg            VARCHAR(255)
-------------------------------------------------
-- initialize
-------------------------------------------------
SET @Proc = 'dbo.proctblAmenitySpec_UpdateForAmenityIDAmenityTypeSpecID'
SET NOCOUNT ON
-------------------------------------------------
-- update
-------------------------------------------------
UPDATE       tblAmenitySpec
SET      Flg = @Flg,
      Qty = @Qty,
      AmenityAttrValueID = @AmenityAttrValueID
WHERE      AmenityID = @AmenityID AND
      AmenityTypeSpecID = @AmenityTypeSpecID
-- handle errors
SELECT      @ErrNbr = @@ERROR, @RowCount = @@ROWCOUNT
IF @ErrNbr <> 0  BEGIN
      SET @ErrMsg = 'failed to update'
      GOTO Proc_Err
END
-------------------------------------------------
-- all is good
-------------------------------------------------
RETURN 0
-------------------------------------------------
-- error handler
-------------------------------------------------
Proc_Err:
RAISERROR (@ErrMsg,16,1)
RETURN @ErrNbr
GO

Here' the script to load data into the table:

INSERT tblAmenitySpec VALUES (1,1,1,1,null)

Here's my snippet of Access XP VBA code which demonstrates the problem:

Public Sub RecAff _
    ( _
        ByVal vstrDataSource As String, _
        ByVal vstrInitialCatalog As String, _
        ByVal vstrUserID As String, _
        ByVal vstrPassword As String, _
        ByVal vfUseProc As Boolean _
    )
   
    Dim lngAmenityID As Long
    Dim lngAmenityTypeSpecID As Long
    Dim strConnection As String
    Dim fFlg As Boolean
    Dim dblQty As Double
    Dim lngRecAff As Long
    Dim varValueID As Variant
   
    Dim cmd As ADODB.Command
   
    strConnection = "Provider=sqloledb;" & _
                             "Data Source=" & vstrDataSource & ";" & _
                             "Initial Catalog=" & vstrInitialCatalog & ";" & _
                             "User Id=" & vstrUserID & ";" & _
                             "Password=" & vstrPassword & ";"
    lngAmenityID = 1
    lngAmenityTypeSpecID = 1
    fFlg = 1
    dblQty = 999
    varValueID = Null
    Set cmd = New ADODB.Command
    With cmd
        If vfUseProc Then
            .CommandText = "proctblAmenitySpec_UpdateForAmenityIDAmenityTypeSpecID"
            .CommandType = adCmdStoredProc
            .Parameters.Append .CreateParameter(, adInteger, adParamReturnValue)
            .Parameters.Append .CreateParameter("@AmenityID", adInteger, adParamInput, , lngAmenityID)
            .Parameters.Append .CreateParameter("@AmenityTypeSpecID", adInteger, adParamInput, , lngAmenityTypeSpecID)
            .Parameters.Append .CreateParameter("@Flg", adBoolean, adParamInput, , fFlg)
            .Parameters.Append .CreateParameter("@Qty", adDouble, adParamInput, , dblQty)
            .Parameters.Append .CreateParameter("@AmenityAttrValueID", adInteger, adParamInput, , varValueID)
        Else
            .CommandText = "UPDATE tblAmenitySpec" _
                         & " SET Flg=" & Abs(fFlg) _
                         & ",Qty=" & dblQty _
                         & ",AmenityAttrValueID=" & IIf(IsNull(varValueID), "NULL", varValueID) _
                         & " WHERE AmenityID=" & lngAmenityID _
                         & " AND AmenityTypeSpecID=" & lngAmenityTypeSpecID
            .CommandType = adCmdText
        End If
        .ActiveConnection = strConnection
        .Execute lngRecAff
    End With
    Debug.Print lngRecAff
End Sub

The dynamic SQL update works and the recordsaffected value from the adodb command is 1 when I call the above vba code with:

call recaff ("(local)","databasename","userid","password",False)

The stored procedure update works but the recordsaffected value from the adodb command is -1 when I call the above vba code with:

call recaff ("(local)","databasename","userid","password",True)

Why do I get -1?