wpf中，xml绑定到datacontext和绑定到普通object 有何区别？

代码

<Root> 
 <Unions>
        <Item FURN_GRP_ID="64" FURN_GRP_NAME="ddd" />
        <Item FURN_GRP_ID="61" FURN_GRP_NAME="沿海地" />
        <Item FURN_GRP_ID="62" FURN_GRP_NAME="地中海" />
        <Item FURN_GRP_ID="63" FURN_GRP_NAME="沙滩" />
    </Unions>
</Root>

2. Xaml里面这样子绑定
<XmlDataProvider x:Key="typeData" Source="Xml/Base.xml" XPath="/Root" />

3.把指定的节点绑定到DataContext和Test
<UC:UCUnionList Grid.Column="3" Margin="27,0,83,10" x:Name="ucUnionList"
                                 DataContext="{Binding Source={StaticResource typeData},XPath=/Root/Unions/Item}"
                                 Test="{Binding Source={StaticResource typeData},XPath=/Root/Unions/Item}"
                                 />

4.UCUnionList的cs里面这样写：

代码

[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
[Localizability(LocalizationCategory.NeverLocalize)]
public object Test
{
   get { return GetValue(TestProperty); }
   set { SetValue(TestProperty, value); }
}

public static readonly DependencyProperty TestProperty = DependencyProperty.Register("Test",
            typeof(object), typeof(UCUnionList));

5.在cs代码里面检测拿到的值

var a = ucUnionList.Test;
var b = ucUnionList.DataContext;

发现这个情况
a.GetType()
{Name = "XmlElement" FullName = "System.Xml.XmlElement"}
只拿到了一个子节点，即  <Item FURN_GRP_ID="64" FURN_GRP_NAME="ddd" />

b.GetType()
{Name = "XmlDataCollection" FullName = "MS.Internal.Data.XmlDataCollection"}
拿到了所有的子节点
 <Item FURN_GRP_ID="64" FURN_GRP_NAME="ddd" />
<Item FURN_GRP_ID="61" FURN_GRP_NAME="沿海地" />
<Item FURN_GRP_ID="62" FURN_GRP_NAME="地中海" />
<Item FURN_GRP_ID="63" FURN_GRP_NAME="沙滩" />

求教各位，这是为什么呢？