Measuring diff on sono?

I went for my second appointment today and whereas officially I am 11 weeks and 5 days (period first day was 8/9/17) and when I was there for my first appointment it measured a day earlier than that but he said all was good and that my due date would be 5/15. Today the sono showed it as 12 weeks and 2 days. Not a major difference and he didn’t comment on it other than that’s what it showed but how is that even possible? I know exactly when my period started? How could it be that I’m later (although few days). And which an I really?