Why is confidence always -1?

Imports SpeechLib
Public Class SpeechClass
    Private _commands As Speech.Recognition.Choices
    Private _gbldr As Speech.Recognition.GrammarBuilder
    Private WithEvents RecoContext As SpSharedRecoContext
    Public WithEvents _recnizer As New Speech.Recognition.SpeechRecognizer
    Private _grm As System.Speech.Recognition.Grammar
    Private vLastCommand As String
    Private vConfidence As Integer = 0
    Public Property ValidCommands() As Speech.Recognition.Choices
        Get
            Return _commands
        End Get
        Set(ByVal value As Speech.Recognition.Choices)
            _commands = value
        End Set
    End Property
    Public Property LastCommand() As String
        Get
            Return vLastCommand
        End Get
        Set(ByVal value As String)
            vLastCommand = value
        End Set
    End Property
    Public Property Confidence() As Integer
        Get
            Return vConfidence
        End Get
        Set(ByVal value As Integer)
            vConfidence = value
        End Set
    End Property

    Public Function Initialize(ByRef str_response As String) As Boolean
        Dim retval As Boolean = True

        Try
            _commands = New Speech.Recognition.Choices
            _commands.Add("Computer")
            RecoContext = New SpSharedRecoContext
            _gbldr = New System.Speech.Recognition.GrammarBuilder(_commands)
            _grm = New System.Speech.Recognition.Grammar(_gbldr)
            _grm.Enabled = True
            _recnizer.LoadGrammar(_grm)

        Catch ex As Exception
            retval = False
            str_response = ex.Message
            If Not ex.InnerException Is Nothing Then
                str_response = str_response & vbCrLf & ex.InnerException.Message
            End If
        End Try
        Return retval
    End Function
    Public Function StartRecognition(ByRef str_response As String) As Boolean
        Dim retval As Boolean = True
        Try

            _grm.Enabled = True

        Catch ex As Exception
            retval = False
            str_response = ex.Message
            If Not ex.InnerException Is Nothing Then
                str_response = str_response & vbCrLf & ex.InnerException.Message
            End If
        End Try
        Return retval
    End Function
    Public Function AddCommand(ByVal str_command As String, ByRef str_response As String) As Boolean
        Dim retval As Boolean = True

        Try

            _commands.Add(str_command)
            _gbldr = New System.Speech.Recognition.GrammarBuilder(_commands)
            _grm = New System.Speech.Recognition.Grammar(_gbldr)
            _recnizer.LoadGrammar(_grm)

        Catch ex As Exception
            retval = False
            str_response = ex.Message
            If Not ex.InnerException Is Nothing Then
                str_response = str_response & vbCrLf & ex.InnerException.Message
            End If
        End Try

        Return retval
    End Function
    Public Function StopRecognition(ByRef str_response As String) As Boolean

        _grm.Enabled = False

    End Function
    Private Sub RecoContext_Recognition(ByVal StreamNumber As Integer, ByVal StreamPosition As Object, ByVal RecognitionType As SpeechLib.SpeechRecognitionType, ByVal Result As SpeechLib.ISpeechRecoResult) Handles RecoContext.Recognition

    End Sub

    Private Sub RecoContext_SoundStart(ByVal StreamNumber As Integer, ByVal StreamPosition As Object) Handles RecoContext.SoundStart

    End Sub

    Private Sub _recnizer_SpeechRecognized(ByVal sender As Object, ByVal e As System.Speech.Recognition.SpeechRecognizedEventArgs) Handles _recnizer.SpeechRecognized

        LastCommand = e.Result.Text.ToUpper()
        Confidence = e.Result.Confidence      '<<< always -1 why?
        MsgBox("I heard " & e.Result.Text)


    End Sub
End Class

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