The proverb uses interesting imagery of the womb being an indiscriminate container. While all children are blessings their deeds and professions are not confined to those of their parents. The process of upbringing is a complex one that can sometimes result in children with traits that are opposite to their parents. This proverb was formed by our ancestors as explanation and reaction to such scenarios. It is warning that parenting is not an easy process which may have other forces combining to produce a different outcome than the intended one.
The proverb is used to explain situations where parents with seemingly good values bear children who end up with bad values and character traits. It is also used to warn people who see parenting as a simple role or view bearing kids as something tantamount to producing angels to be cautious as many possibilities may arise.