WEATHIER, HEALTHIER, PEOPLE HAVE MORE SONS - Article here and there are many others out there that discuss this phenomenon.

Better health, socioeconomic status, etc. are all correlated with your children doing better. If you want to compare the effect sex has on male and female performance, you need to account for socioeconomic status of them that causes differences, which I have never seen done before. "For billionaire fathers, the odds of having a boy are 65%.", "We now know that bad weather makes for more baby girls, as does fasting for Ramadan or suffering from morning sickness" It should be easy to see how that can trickle down to men having better outcomes than women, simply because of who they are born to. If you want to find a difference caused be gender, you need to make sure you aren't measuring socioeconomic differences.

I can not find any studies looking at the socioeconomic situation of parent's comparing boys and girls, to see how significant the difference is, just ones that mention richer people have more boys. Best I can find is game theory, or things only looking at billionaires or other select groups, nothing that looks at the trend across income levels.

Also, if someone knows of a study that does factor in socioeconomic status on the gender gap, please send it to me. All sorts of studies and articles I've read about it never discussed this. 1 2 3 4 these are just 4 more I looked up while writing this to show how they don't mention it, I've read many more.