How can you not know how far you are?

Vasa • Nature lover, reader, cheesemonger
First time pregnant, and I'm confused at how many women are surprised when they find out their baby's gestational age. As long as you know when you had your last period, shouldn't that narrow it down to the right week at least? And if you took the step of tracking your cycles/temps/days of intercourse, seems like you'd know when conception was within a couple days.