last period date and initial scan is just an estimation .. it all depends on your ovulation, if your period is regular/irregular and other factors as well .. a slightly more accurate one will be when you have your 20th weeks anomaly scan where they do a thorough detailed on the baby .. from the measurements it might be a different gestational age again . :)
The gestational week can change / difficult to date in early pregnancy, it’s very normal. Soon your gynae will confirm your EDD when they can capture the proper length of the baby