This sounds normal. I had it explained to me that you might know your LMP but the egg might have taken longer to get down the tube. There are all sorts of reasons for it being out.

I said my LMP was 20/10 (this is as close an estimation I could think of), which would mean I am 15+ weeks. But in my amnio today the baby is only 14 weeks old, so I'm out by quite a bit without meaning to be so.

Jordie, I don't think you have anything to worry about. But that is my non-medical opinion, which I like to give regularly.