Confused on dates

So I found out I was pregnant on the 20th July. I went to drs on the 26th for bloods and confirmed. The doctor said I was five + weeks. My last period was the 26th June. So according to my period I would be 4w5d but according to Dr I'm over 5w and when I worked out when I had sex I would have conceived on the 3rd or 4th of July and that would put me at 5w5d. I haven't had my dating scan yet to confirm how far along. Just wondering if anyone can help me understand please?