Gestational Age verses time of Conception

Question. When an ultrasound is done in the first trimester and it says on the ultrasound picture GA=8wks and gives a percentage next to it, this means it has calculated that the gestational age of the fetus is 8weeks along correct? So my big question is, if the ultrasound is calculating gestational age that means conception had occured 2 weeks later than the Gestational Age, so in this case fertilization age would be roughly 6 weeks?? and the follow up to that question is, if fertilization occured lets say 2 or 3 weeks earlier or later than that, the gestational age calculated by the ultrasound would be different too right? essentially, the time of fertilization would affect the calculations of the ultrasound's gestational age dating ? i know that the ultrasound is accurate to within 6 days in the first trimester, so I just want to make sure that if fertilization occured more than that in time difference the ultrasound would reflect that.
Thanks so much!