WPF Boolean Attached Properties and the property changed callback

c#:

namespace AttachedProperties
{
   public class MyNominalObject :DependencyObject
   {
       public static readonly DependencyProperty UseMouseLeaveProperty =
           DependencyProperty.RegisterAttached("UseMouseLeave", typeof (Boolean), typeof (MyNominalObject), new PropertyMetadata(OnUsingMouseLeave));

       private static void OnUsingMouseLeave(DependencyObject d, DependencyPropertyChangedEventArgs e)
       {
           var result = d.GetValue(UseMouseLeaveProperty);
       }

        public static void SetUseMouseLeave(UIElement element, bool value)
       {
           element.SetValue(UseMouseLeaveProperty, value);
       }

       public static bool GetUseMouseLeave(UIElement element)
       {
           return (bool) element.GetValue(UseMouseLeaveProperty);
       }
    }

xaml:
       <ListBox local:MyNominalObject.UseMouseLeave="True">
            <ListBox.Items>
                <ListBoxItem Content="1" />
            </ListBox.Items>
        </ListBox>    


Question:
Why is a breakpoint hit on " var result = d.GetValue(UseMouseLeaveProperty); " when the xaml is local:MyNominalObject.UseMouseLeave="True",

but the breakpoint is not hit it the xaml is:
 local:MyNominalObject.UseMouseLeave="False",