the backgrounds they show are suggestions. They are giving you one possible reason for those stats, for those who need guidance. A noble might be more charismatic due to training on how to influence people. Some one else might just be born that way, yet another's mother may have used a wish for a charismatic daughter.
the old system limited your starting stats by race. Suggesting NO level 1 orc could be as dexterous as the most dextrous elf, no matter their birth, training, plane, magic etc.
the old system didnt say what is common, it said what was possible. In the world and setting of dnd, its unlikely that there aren't exceptions to any norm.
What is normal, and what is possible are two different things
