Why NgModelController.$modelValue != $parse(attrs.ngModel)(scope)?

Code: http://plnkr.co/edit/xPZM5E7tjYqlt5NIabIu?p=preview line-no:17

In this code if I use ctrl.$modelValue = nVal; instead of $parse(attrs.ngModel).assign(scope, nVal); then it does not work. Can you please point-out the reason?

angModule.directive('moChangeProxy', function ($parse) {
    return {
        require:'^ngModel',
        restrict:'A',
        link:function (scope, elm, attrs, ctrl) {
            var proxyExp = attrs.moChangeProxy;            
            scope.$watch(proxyExp, function (nVal) {
                if (nVal != ctrl.$modelValue) {
                    //ctrl.$modelValue = nVal;  // This does not work                  
                    $parse(attrs.ngModel).assign(scope, nVal); // This works well
                }
            });
            elm.bind('blur', function () {
                var proxyVal = scope.$eval(proxyExp);
                if(ctrl.$modelValue != proxyVal) {
                    scope.$apply(function(){
                        $parse(proxyExp).assign(scope, ctrl.$modelValue);
                    });
                }
            });
        }
    };
});