dating scan question

my due date is july 11 so based on that my probable date was october 18 give or take… on december first i had my dating scan and was told i am 8 weeks 2 days (irregular periods and last period was sept 10 so couldn’t go off lmp bc it would make me farther along) but when i count from the 18 to the 1st it is only 6 weeks 2 days so where did the extra 2 weeks come from ? now i feel like i was told the wrong dates . someone explain pls because it’s confusing me to this day.