Tulip like Kelly said there is a margin of error from 7-10 days for scans. But everything has a margin of error too.

When I was PG with Matilda, but my LMP I was due 30th May, my first scan put it at 1st June and my 2nd put it at 3 June. I was induced on the 16th and she was born on the 17th.

Sometimes a scan can be taken during a growth spurt as well, which is why they say the earlier the scan the more accurate.