^Yeah, I agree with that assessment. Perhaps it's as simple as different Timelords having different gene combinations. It could be like a Punnett square, with Male regeneration being the "dominant" trait. The Doctor is MM, people like the Corsair are Mm, and some Gallifreyans are only ever female and are mm.
Some of them will always be male. Some will always be female. But there are those out there that have a 50/50 chance every time they regenerate of being either male or female.