The cultural assumption that women are the weaker, gentler sex is alive and well. People still are prone to find it inconceivable that a mother could harm her child even though the stats have been clear for decades that they are responsible for around 1/3 of the murders of children under five.
Cops should know better, but it is hard for training to entirely overcome years of social indoctrination.
Probably because women are half the population but are responsible for only a small fraction of violent crimes. Even your statistic paints them in a favorable light: if women commit 1/3 of the murders of children under five, men commit 2/3.