Genetic predisposition because fem seed are made by causing A sexuality ina fem to coax out the herm so you can then self pollenate a clean plant with no induced stress. Then grow those seeds out isolating any bad genetics out by staying dialed in, on the hunt for any dudes that look like a lady…..rinse and repeat. The more times you do this successfully, the more stable a strain becomes. But in fems case, there is ALWAYS the off chance one pops up like a different phenotype does.
It’s genetic, like kids. They are never the exact carbon copy of the parents. There is even recessive genes too, like when two blondes have a red headed kid. It’s part of the deal.