Why have I gone OVERDUE when my DATING SCAN said I was 12 weeks?

So by my dating scan which gave me a date going by my babys measurements, i am 41 weeks pregnant this wednesday so that gives me a due date of 20th april...which was my actual due date. So I'm overdue now and I dont get it because i know my conception could have been off but my EDD wasn't going by my Last period it was going by my BABY'S measurements..so y have i still gone over due?