Early ultrasounds are very accurate to within a week. I too came closer to my LMP but think he was a little overdone!! He was 9 pounds and 22 inches long!! The doctor will tell you that an ultrasound prior to 9 weeks is accurate to within a couple of days. Congrats!
Welcome Mima09!
As for your question about ultrasounds. Well, I based off my LMP was due Nov 29th, I then had an u/s at 8/9 weeks pregnant and they bumped the date to December 3.
Turns out my LMP date was right, my gorgeous little guy was born on the 29th!
I think it's also pretty rare to have your baby on it's due date, like 4% chance or something.
Anyway, it's all a guessing game...they don't really know, and you won't know until the day of. Trust me, I had so many days I thought it was the real thing and it was just braxton hicks.
All I can suggest it take the due date lightheartedly otherwise if you go beyond it you'll be so frustrated and not enjoying the last few days/weeks of pregnancy.