The further along you get FJ, the less accurate the estimated dates become.
i wouldn't worry yourself with any dates from your 18 week scan, they are not as accurate as either your period dates or an early scan.

You will notice on the images, different measurements that give different dates, for eg, LOF- length of femur will give one date. BPD- biparietal diameter will give another. A series of these measurements are taken and then an average due date is calculated. But you might have a baby with long legs, or a bigger head and this will change the estimated due date.

Like you say- i wouldn't read anything into it! How exciting, getting close!