Due date :/

So I'm 16 weeks pregnant with my second Baby, but the more I think about my due date the more I don't think it's right... I know they go by size at the scan, basically, they are saying we conceived on Christmas day, but I know I had a period a week before and finished my period roughly 3 or 4 days before Christmas. Therefore I should technically be a week and a half ahead of where they have dated me... Has anyone had this before and have you been right or where the dates right? Or is it just me? Or am I even making sense?? 😂 Edit-my scan was at 12 weeks sorry I should of said that to begin with 🙈