Okay, here's the official deal.
If you buy into a culture, regardless of how you were raised - you get that cultural bonus. Culture gives you, at times, a huge rp advantage for survival and privileges to locations you would normally not have. The sea of grass and the Drykas are one of the biggest examples I can think of. If you aren't Drykas and go there - odds are you are toast. Totally. So, regardless of where you start or how you were raised you MUST take your cultural racial bonuses rather than just Human bonuses the +15. Think of it this way.. you're purchasing a culture for sacrificing 5 skill points - regardless if you use it or not.
Being just a plain human can be a disadvantage - you are basically starting out as an average joe. They have no 'cultueral heritage or homebases' and are vulnerable in a lot of places... that's why we give them the +15 to anything. There are certain 'cultures' that also get a plus 15, though that was not my doing as a Founder. All the cultures I created give a +10 because their location is their advantage. Everything plain humans enjoy boils down to being - built recently and just forming. I imagine, in another five hundred years, Syliran will be a human culture.
Right now, it just speaks of lack of bloodlines and a hodgepodge of racial mixes.
Hope that clears things up.
Goss