There is no "I" in "You"

[Aoi Magi]

Well I am me because of my choices and the social and environmental conditions I've been raised in. We have a lot of factors effecting us during our lifetime and the factors are so numerous that in total they are unique for each individual.

But from your questions I think you are making a mistake in thinking that we are more than our physical biological selves. You asked why someone is born to particular parents, but the thing is there is no 'someone' before they are born. Existence begins at birth. So the question shouldn't be why someone is born a certain way, rather it should simply be why someone is what they are.

I'm simply asking atheists what mundane factor(s) determined their particular experience of self in distinction to roughly six billion alternate, extant possibilities.

You honestly expect me list out an infinite number of factors like that? C'mon, just a basic combination of your parents, friends, relatives, and their interactions with you in any given moment makes that moment unique to you. Our lives are made up of countless such unique moments which makes us unique individuals.