Its a survival mechanism I reckon. Not sure why it happens. It does. Though.
Even with the most stable strain cannabis can throw pollen to pass it's genetics on.
I'll have to dig to find the science behind it.
I do know that no fem breeder, that's respected anyways, will claim 100% female offspring. Usually they claim 99 or 99.9%.
Life will find a way.
The one to ask if I remember right is
@Dr. Who .
Everywhere I've read it states like one in a thousand.
That doesn't mean that it is for every thousand. You may run several thousand and not see one.
Then again you may run a few hundred and see a few of them.