Hun I had the same thing with ds. The scans aren't always perfect, and sometimes you can ovulate at funny times, iykwim. Most doctors and midwives will go off the date on the scan (and you will be asked "Are you sure about your dates?" about 100 times )

Nothing to worry about. for the record, ds was 4 days early according to the scan, but according to all the midwives he 'looked like' an overdue baby so who knows?