i was two weeks off from my estimated due date based on my last period...I ovulated later.
The conception date could have been off.
It's possible that going by your last missed period alone wasn't a good enough indicator on how far along you are. more than likely baby is not as big as they would be for 13 weeks, so they changed your due date basically. It's nothing to worry about, but you should ask your doctor about the reason behind it if you really want to know the exact reason.