The scans really assess the conception date rather than the arrival date as the baby can be born either side of the "due date" as others have said.

I know when both of my girls were conceived and worked out the "due date" from there - the scans showed a "due date" only one day out from our dates so they were pretty much spot on for working out conception dates.

DD1 was "due" on Feb 26th but arrived 15 days earlier on Feb 11th. DD2 was "due" Feb 22nd and arrived on the 19th.