Male patient gets operated on: outcome does not vary between male and female surgeon
Female surgeon operates: outcome does not vary between male and female patient
M on M = F on M = F on F
So far, so good...
Male surgeon operates on female patient: worse outcomes all around...
Either male surgeon's fat fingers are too rough for the delicate innards of female patient, or the bloody male ego gets in the way, and their misogyny starts shining through their actions in small, but statistically significant ways (they botch the job because they have less respect for the patient)
One way to distinguish between the two hypothesis would be to analyse the data in term of patient size (and surgeon size).
Note: the idea that women's body are just too delicate sounds bollocks. Female bodies are in fact remarkably resilient, hence the higher longevity. Consider for instance survival rates for COVID. It's not an isolated anomaly.