Chances are they cracked the screen replacing the battery. It's super easy to do given how thin the screen is. They probably didn't want to pay the cost, so they said it was cracked. :)
Is your defect next to the volume up/down button? That's where mine's at and it's only 2 days old. I'm debating whether to send it back or not. It's really small, but at the same time it's not going to get any better over time...