@Suiseiseki while mutation is also a way for this effect to occur, the process i explained is literally just a very common mishap when combining the parents' genetic information just happening to affect this specific gene

in any case chromosomes carry a lot more information than just the genes responsible for sexual dimorphism so an x chromosome is still an x chromosome even if it includes something you think belongs in a y

also a neat fun fact: some research has suggested that this condition is actually *far* more common than is frequently assumed but most cases go undiagnosed because it has no negative health effects and requires expensive labwork to confirm