Fetal development is not really that consistent, to guess it down to a week or two based on physical appearance. Anything from 37 to 42 weeks is considered a “normal” pregnancy length. That means someone oversimplifying things could say any milestone might be +/- a couple of weeks. Edge cases might move the length of an otherwise-healthy pregnancy down or up an entire month.

Even implantation isn’t that consistent. I’ve heard that sperm can linger for something like 5 days before implantation occurs. The whole middle-school health class version where the sperm swim up and race to the egg is kind of total nonsense.