Dating scans are most accurate early on. All eggs start off at about the same size and then things like genetics start to come into play as you get further along and the size of babies begins to vary as they develop. But early scans can be inaccurate if the person doing the scans measures slightly off the sac. So it is possible your early scan was slightly out, but the date taken from the early scan is more likely to be accurate than your scans done at 19 + weeks.

When was the second ultrasound done? if it was just a week or two after the first, then that is most likely your most accurate measurement from a scan.....