With group A, an accidental pregnancy does not necessarily mean that the person never wanted children, just that they didn't want to get pregnant right that moment.

As for where the desire for children comes from, what you're asking is somewhere along the lines of asking why a person is gay or straight or why they're sexual or asexual. Some people have a lot of the wiring for wanting a baby, some people have a little, and some people have none. People's experiences through life can impact how they act on that, but that's different.

Once an actual child is born, we can assume that the person (in the developed world, usually) decided to go through with the pregnancy and have a baby which is a positive sign that they want children at least a little. There are a lot of powerful hormonal and wiring forces at play that make you bond with and love your children.

Your husband probably drives you fucking crazy sometimes, and being in a relationship requires compromises, and you'd have more privacy and independence without him, right? But the bond you have with him and the fulfillment you receive from that relationship outweighs the negatives.