The correllian (and likely) mandalorian human entries are for a defined culture. It is more sociological than biological.
Think of it this way; the default human is meant to represent any human from anywhere in the galaxies multitude of human cultures. As a default option in every game, you can easily use the human to make a character from Alderaan or Chandrilla, to Kuat or Kanjiklub. The free non-career skills are meant, as I see it, to make sure your characters culture - whatever that may be, is represented. The Corellian and likely Mandalorian variants are meant to be a defined culture and taking to heart the common traits of that culture.
Basically, if you want to play a stereotypical Corellian or Mandalorian play the sub-species. If you don't, play default. The three ranks at construction is nice, especially if you like to spend more xp on talents and skills than the oft touted best way to play; boost your attributes.