Here is the most likely reason for this statistic. (Of you can call it that since it is objectively statistically insignificant, being that male doctors treated 150X more women overall).
The male doctors are better doctors (all factors such as age and experience being the same), and its well documented that they can handle higher stress levels and make Less emotionally influenced mistakes. So they get the tougher cases to begin with. This is a plausible of an argument as any in the article or mentioned here.
Is funny that those who fight gender biases the most are some of the most biased.