Doesn't have to be, Could be a mistake on the seller's part, no matter how much they were charging. If the seller realises her mistake, and asks for it back then I'd give it back, I'm not sure if there is any legal obligation or anything like that but it is morally wrong.
If it truly is a factory mixup, Congratulations and you've got yourself some extra space.
As for everyone who is saying "It's the Sellers fault" etc, Yes they may have have made a mix-up but if you were in that position, then you know - you'd be pissed off as well. You never know the situation that the seller is in, might be selling it to feed their kids etc.
But again, if the seller asks for it back, I think you should return it - but if it's a factory mistake then Well Done.