In WPF, some collection properties require you to specify an object element for the collection itself, e.g.:

    <Something>
      <Something.Children>
        <ChildrenCollection>
          …
        </ChildrenCollection>
      </Something.Children>
    </Something>

Other collection properties require that you not specify an object element, e.g.:

    <Something>
      <Something.Children>
          …
      </Something.Children>
    </Something>

Other properties, most notably .Resources, allow both syntaxes.

Why is that?  We've changed the rules of couple times over the years, but these days there's nothing magical about implicit and explicit collection tags, rather the behavior falls out of the xaml spec.  The first syntax happens when you have a collection property that can be set and defaults to null:

Private ChildrenCollection_children = null;
Public ChildrenCollection Children {
    Get {return _children;}
    Set {_children = value;}
}

The second syntax happens when the collection property provides a default collection object (the preferred way of doing things).  And the third syntax (either syntax allowed) happens when you have a settable collection property that has a non-null default value.