According to my LMP I was 12w5d when the did my ultrasound I was11w3d. Your LMP can't always say your EDD but the ultrasound is a lot more accurate when figuring your EDD. ( and to be honest when the doc told me my first EDD of may 15th I told her she was wrong and I couldn't be that far along and thats even knowing the exact date of LMP) some times it can take a while for the egg to actually be inseminated and stick to the wall. Just take it as you have a little more time to plan and don't stress/ worry about under development. Your baby is just fine and ask questions when you can. But as long as your doing everything you can do right your baby is going to be fine.
If your baby is measuring fine in all other ways (heart rate and developmental markers) perhaps you simply had an unusually late ovulation in the month when you got pregnant.