Tim Harford ("The Undercover Economist") suggests that it is due to selection bias:
...in a country where the barriers to paid work for women are high, the few women who do have jobs at multinational companies are outstandingly good. These high‑flyers are promoted and paid more than the average man. This suggests that there are women outside the workforce who, if they did have paid jobs, would be well above average even if they weren’t superstars. If the barriers to their workforce participation could be lowered, they would raise the productivity of the companies that employed them.Bottom line: Inefficiency implies opportunity for those with the imagination and initiative to exploit it.