AS3 setter question: model.selectedMetric = x; where x already is the selectedMetric. The setter is NOT invoked. Why?

ModelSnippet.as

        private var _selectedMetric:Metric;

       [Bindable]
        public function get selectedMetric():Metric
        {
            return _selectedMetric;
        }

        public function set selectedMetric(value:Metric):void
        {
            _selectedMetric = value;            }
        }