In my experience, women are much less likely to be "cowboys". Alternatively, I've seen few women surgeons routinely attempt riskier corrective surgeries. It could be more about patient selection than skill.
In the study, 1,000,000 patients were treated by a male surgeon, and 100,000 treated by a female surgeon.
That's quite the discrepancy. Doesn't explain the results, but it does show that there were far more male surgeons than female. Which might mean that there is a selection bias somewhere in the process.