Have you guys ever tried to redraw an iMac or other Apple hardware? You might have noticed that you can't just draw a rounded rectangle, the corners aren't actually quarter circles. Instead they taper off gradually before becoming flat.
Well, now that Jony Ive is in charge of iOS, he's brought that attention to detail to the home screen. Not only has the rounded corner radius and size of the icon changed, but also the method by which its corners are rounded.
Be careful when doing complicated edge effects in this new and magical world. They might not come across exactly as you intended.