If the firstborn is a girl, the parents might try to have more kids until they have a boy. And if their family registry already says they have a kid, they might run into some bureaucratic issues that might prevent them from having more kids.

If their firstborn is a boy, they will just register him. And if they choose to have more kids after that, be it a girl or a boy, they are forced to leave them off the family registry. In this case, since they already have a son, they might not be too worried about running into issues.