Isn’t it plausible that one rationale for this is the same as was reported a few months ago about how “the nations with the least gender equality, as determined by the World Economic Forum’s Global Gender Gap Report, had the highest representation of women in STEM” (https://www.wsj.com/articles/why-arent-there-more-women-in-science-and-technology-1519918657).
That was explained by one commentator in that article as being because “if girls expect they can ‘live a good life’ while working in the arts, health or sciences, then girls choose to pursue what they are best at—which could be STEM, or it could be law or psychology…However, if the environment offers limited options, and the best ones are in STEM, girls focus there…”
In the highest income districts, many girls probably can expect to ‘live a good life’ and their parents have the money to allow them to pursue what they want and do best at (and they are clearly doing relatively better in arts subjects). Majoring in ballet (like my D18) or philosophy in college is not likely to be the choice you’d make if you were impoverished and wanted a better life. Majoring in engineering or medical fields is likely to be a better route out of poverty, just as it is in low income countries like “Algeria, Tunisia [and] Albania”.
Remember this is not saying that the top boys are better than the top girls, it is saying that the average of the boys is better than the average of the girls. If the mid ranking girls choose to do their preferred arts subjects (because they can afford to do so) and so don’t focus on math, while the mid ranking boys stay with STEM (because they are not very good at writing) then the average achievement of the boys in math will be higher than the average achievement of the girls, even if the top performers (who are focusing on STEM) are at the same level.