It's normal for them to be off, it's really a guess from the missed period. Most likely it'll change again as your pregnancy progresses.
I had the same thing. Was off by 2 weeks. Its just that i ovulated late. Nothing to worry about.
My due date as per scan verse my last perios was 3weeks off! As long as bby is growing correctly, dont stress x
You go by ultrasound , not lmp. After that first sonogram that means nothing