We are all born differently. Our brains work different ways. Our muscles want to do different things. As such we have a propensity towards one thing or another.
It is true if you have kids you can see this. Every single baby is completely different, even newborns, in how they respond to things.
When discussing populations, and making statements about them, the presence of an outlier does not invalidate the statement about the characterization of the population.
We are, of course, all different. Diversity is a great strength. That doesn't mean that there's a biological basis for every kind of difference.
Certainly not, but I also don't believe that we are all born with the same abilities. Things that comes easy to some are more difficult to others. Even at the youngest ages kids show signs of what they like and don't like, and this will play out later as ability.
I can see how it would be disappointing if you didn't have two children growing up in the same environment and the differences between them is the same as between night and day.
Not that I'm saying these type of studies are not useful.
As tokenadult points out, these studies are serious business. Anecdata doesn't really cut it.