private void OnTouchDown(object sender, TouchEventArgs e)
    {
        if (m_nCapturedTouchDevice == -1)
        {
            // THIS RETURNS THE RIGHT POSITION :
            m_ptTouchDownPosition = e.GetTouchPoint(this.Parent);
            // Capture the touch
            this.CaptureTouch( (uint) e.TouchDevice);
        }
    }

    private void OnGotTouchCapture(object sender, TouchEventArgs e)
    {
        // Handle a pointer capture

        // THIS RETURNS A WRONG POSITION : 
        var ptWrongPosition = e.GetTouchPoint(this.Parent);

        OnPointerCaptured(m_ptTouchDownPosition);
        // Set the captured touch device
        m_nCapturedTouchDevice = e.TouchDevice;
    }