27 day cycle lmp6/6 first us 7/23. 6w1d nxt us 8/13 9w1d how could i hv conceived on 6/22. doc said conception wad around 6/17?

Due date: Expected due dates are assigned by first day of last menstrual period, or may be changed by a first trimester us if dates are off by more than 7% from the lmp. First trimester us is extremely accurate (within 4-5 days). Once a due date is established, it does not change. The future us's are compared to the earliest us or LMP due date to determine if baby is growing adequately.