So why does it happen?

In the 1970s, Dr Julianne Imperato-McGinley, discovered that they have a genetic condition that causes a deficiency in an enzyme called 5-alpha-reductase, which converts testosterone into dihydro-testosterone.

Thus, male external & internal genitalia do not develop despite the presence of the Y chromosome & SRY gene, and they appear female when they are born.

At puberty, a surge of testosterone causes their body to develop internal & external male genitalia alongside secondary male characteristics.