I wonder if this has anything to do with kids learning how to work around their own parents' parenting style. They'll have had years to learn how to disobey their own parents by the time this woman comes into the picture. If her style is different enough, the kids won't know how not to behave.

Then when she has a kid, her kid learns exactly the mental tricks needed to be able to disobey her, by virtue of spending 24×7 with her.

Nature equips kids with instincts, one of which is to be selfish, the others of which can be exploited by parents to override that instinct. But nature also equips kids with a brain, and it is only natural that selfishness uses the brain to learn how not to be manipulated by those other instincts. But that takes time; hence the above results.