Aua EDD vs LMP EDD

Jasmoray

Hello moms I’m a little confused I went in for an ultrasound today & according to my LMP by EDD is 3/21/22 but the tech measured my baby at a week ahead and the AUA EDD is 3/15/22. I’m a little confused because that 3/15/22 would probably put me at a time where I wouldn’t be ovulating. I’m not sure how all of this work. Or is it just because the baby is big that the due date might change?