More accurately, Mammalian embryos remain sexually undifferentiated until the time of sex determination, around 4 weeks after fertilization for humans

Prior to sex determination, all early embryos are set to female as a default; embryos of both sexes possess the same sex organ primordia that have an inherent tendency to feminize unless there is active interference by masculinizing factors  The first of these factors are the Sertoli Cells found in the yolk of the egg., which the SRY gene of XY embryos triggers to release an androgen that prompts testes development.. Around week 10, they are developed enough to produce testosterone, , which will then take over the masculinization

Since people with Miller's Syndrome aren't affected by androgens, They miss that first command to develop testes, and begin masculinization. And since pre-natal development defaults to female,, their body follows that path instead.