Why do sonographers say we can't guarantee the sex of your baby/ies, even tho in the image you can clearly see what it is?

Would the penis drop off just before birth on boys to make girls, or is the baby able to tuck its penis between its legs to make a vagina just to trick the sonographer?
Not that we know the sex yet its still too early , but was reading up about it again and that is something that always pops up .