Multibinding ConvertBack doesn't pass the correct value?

There is a very strange problem which is confusing me a lot.Just like the code below,I had created a [Button] and multibound its [Canvas.LeftProperty] to [Entity.X] and [Entity.Z].The [Entity] class has implemented the [INotifyPropertyChaned].

<Canvas><Button x:Name="btnTest"><Canvas>

private void Binding()
{
  var enity=DataContext as Entity;
  var multiBinding=new MutiBinding();
  multiBinding.Mode=BindingMode.TwoWay;
  multiBinding.Converter=new LocationConverter();
  multiBinding.Bindings.Add(new Binding("X"));
  multiBinding.Bindings.Add(new Binding("Z"));
  btnTest.SetBinding(Canvas.LeftProperty,multiBinding);
}

public class Entity:INotifyPropertyChanged
{
  private double x=0d;
  private double z=0d;

  public double X
  {
    get{ ...}
    set{ x=value;....}
  } 

  public double Z
  {
    get{ ...}
    set{ z=value;....}
  } 

  //INotifyPropertyChanged Members...
}

public class LocationConverter: IMultiValueConverter
{
    public object Convert(object[] values, TypetargetType,object parameter, CultureInfo culture)
    {
        return (double)values[0]*(double)values[1];
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
       return new object[]{ (double)value,Binding.DoNoting};
    }
}

It works well in Convert() Method, [Entity.X] and [Entity.Z] are correctly passed to the [Canvas.LeftProperty].

But the problem is:when I changed the [Button]'s location with Canvas.SetLeft() method,the ConvertBack() methods was fired,but the correct value was not passed to the [Entity],the [value] in [Entity.X]'s set section seemed to be the old one all the time.

Anyone can help?Thank you for your time!

PS:I found a similar question ,but it was not solved either.. :(