6 days difference between my EDD & scan due date

Shannon
Hi Ladies,
So I had my first scan today and we got to see that amazing little heart flickering away at 158bpm. Based on my I am 7w6d pregnant..... However when I had my ultrasound the sonography has my due date 6 days later which only makes me 7 weeks! 
The reason for my confusion is that I did a pregnancy test the day my period was due and thought I was already 4 weeks at this stage.... So based on the sonographer I got a BFP when I was 3 weeks pregnant and about 7 DPO??
Talk about confused!
Has anyone had their dating scan throw them this many days out but then change again at a later scan?