I would say a vaginal ultrasound is more accurate than abdominal one in early pregnancy. But it’s usually a range and an estimate length. The fetal length will also tell you which gestational age you are currently at. If it differ a few days, then shld be ok. If it differ a lot , I think that would be a concern .
Ultrasound scan are usually just an estimate so it might differ. That's nothing much to worry about since gynae says it's alright :)