I get the sense that it's dependent on your POI. It isn't an official interview by any means. I exchanged e-mails with this professor before applying, but I've never met him in person, so he probably just wants to get a better sense of who I am.
From what I can tell, most schools don't do Skype interviews, and even those that do leave it up to the individual POIs as to whether or not they want to interview prospects. So don't sweat it!