Why do I need to write ::shadow after :host? - polymer

Question

I'm looking for an explanation as to why I would need to write :host::shadow in order for my inherited element to style elements in the shadow tree of my base element.
In the following sample code, I would like to be able to style the label for the complete element:
<polymer-element name="my-foo">
<template>
<style>
:host label {
color: red;
}
</style>
<label>My label: </label>
</template>
<script>
Polymer('my-foo');
</script>
</polymer-element>
<polymer-element name="my-bar" extends="my-foo">
<template>
<style>
:host label {
font-weight: bold;
}
</style>
<shadow></shadow>
<input />
</template>
<script>
Polymer('my-bar');
</script>
</polymer-element>
<my-bar></my-bar>
http://jsfiddle.net/Lz0Lcrx3/
The above example makes the label appear red but not bold. In order to fix it I need to change the style on my inherited element to include ::shadow.
<style>
:host::shadow label {
font-weight: bold;
}
</style>
http://jsfiddle.net/Lz0Lcrx3/1/
I know I'm dealing with two shadowroots in this case and I guess the ::shadow is to make sure we are applying the style to all of them, but I figured that :host would've been enough.
Can someone offer an explanation?

Answer 1

After some more searching I found the following article that I think explains this:
http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom-301/
From what I understand, the <shadow></shadow> insertion point will make it necessary to use ::shadow from another shadowDOM tree since we are in fact crossing boundaries. Shadow boundaries that is, not element boundaries.