Measuring behind?

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I’m 8+2 per my last period, but measuring a week behind at 7+2. Does this mean I implanted later than I thought? I’m trying to explain to someone when I could have conceived, and the date for 8+2 makes perfect sense because I know I ovulated during that period of time (I can feel ovulation due to my Endometriosis). Do people often measure behind and then in a few weeks measure correctly?