alt Hacker NewsIt is quite common for babies to come out of the womb with blonde hair, only for it to darken to brown later in life. The baby isn't blonde, it just looks blonde right now.
Same with gender. Doctors observe a flavor of genitals, make a reasonable assumption, and legally assign the gender which seems appropriate.
Not just that, start talking hundreds of millions of births and you get clerical error.
Which could then become rather complicated if there are laws saying the sex assigned at birth has significant lifelong legal consequences.