Actually, i've never seen her head from below, to see if there is some neck... or something else below the head.
Still an error. There's been many times where we've seen her head being held, and never have we seen her have an actual neck. It would actually be weird if she had a neck since many of her interactions wouldn't have worked as well if she had a neck (without it, her head is well rounded in its own way).
As Rath said, her neck is supposed to be in an alternate dimension (explained in a later chapter).
It's explained that her neck exists in (or is) a wormhole between her head and torso. For some reason, this wormhole is perceived by humans as that weird fire thing, while the underside of her head appears to just be skin.
