French person here, this is the problem between Equality and Equity.

For all intents and purposes, we tend to think of ourselves as a very 'Equal' society (Edit: Gosh we can be too proud of ourselves sometimes). Problem is that Equality would be the best thing ever if all other things in life were also equal, but they just aren't. Social connections and a parental success really go a long way in an 'Equal' society. Think about it this way, if every child gets the same K-12 education, what makes the difference between applicants? Which kids have an advantage when figuring out who to contact for applications or how to study for entry exams? Equality benefits some more than others.

Edit 2: I would hope that the Father =/= Parent discrepancy here is due to the low volume of females in these schools >~30 years ago. Maybe n is too small for accurate stats? Additionally this is right around the time when highly educated women in France kind of had to pick between families and careers (sigh), so maybe these women had less offspring than their male counterparts?