XPath-like \, node sequences, and attributes

I don't understand how \ works with node sequences and attributes. Consider first

 <a><b/></a><a><b/></a> \ "b" // NodeSeq(<b></b>, <b></b>)

Ok, \ works with node sequences.

And

(<a b="c"/>) \ "@b" // c

Ok, \ works with attributes

But now consider

(<a b="c"/><a b="d"/>) \ "@b" // NodeSeq()

Huh? It doesn't work with attributes and node sequences?

FWIW, \\ works ok:

(<a b="c"/><a b="d"/>) \\ "@b" // NodeSeq(c, d)

Is this a bug or a feature?

Thanks,

Cay