The 50% who get killed lowers the reproductive fitness of having a girl by the exact amount that the reproductive fitness of having girls is raised by the lower percentage of them relative to men.

That's why I said the women who grow up have higher fitness, not all women born.

The detail of my example, that girl babies are born and then killed, is easy to modify. Imagine a drug that, when taken by a woman before sex, selectively kills XY sperm. Or a sex-selective early abortifact.

The average number of children by men and women must remain equal, but the average number of men children and woman children doesn't have to.