At one point with my last child, who is now 13, he was 2 weeks behind and then at my appt at 24 weeks, he was measuring 3 weeks ahead. It all varies. Some are right on, but they don't have to be. Good luck to you
I shouldn't worry about the scan date being different from your lmp date. The difference can vary up to as much as 2 weeks. As long as your baby is showing growth each week then everything is fine. :)