[tzs]

It's not just about XX or XY. It is about many other biological features that are correlated with XX and XY, but not 100% correlated.

Besides the big one (penis vs vagina) there are things like the rest of the reproductive system, breast size and functionality, brain structures, muscle function, bone structure, facial features, and others.

These develop at different times during fetal development, and don't have to all come out "male" or "female".

In programming terms it is like XX/XY is a master setting for a complex distributed system, and then there are a bunch of flags that control implementation of that master setting in various subsystems of the distributed system which are "supposed" to all be set if the master setting is XX and all clear if it is XY, but sometimes you end up with some set and some clear.