Well what else did they say? Was there a heartbeat? It is actually very easy to be off in weeks. Because the way pregnancy works, they dont go by when you ovulated or had the sex that got you pregnant. Normally, they go by your LMP - weird, and I will never understand why. For example, when I found out I was pregnant, they determined me to be 8 weeks. I had figured about 11 weeks.
Without knowing what else they said about the baby, etc, it is hard to pass on too much advice. All I can say is that I hope it all goes well. Please update us often and let us know what is up okay?