Your genetic makeup is innate. Your upbringing is not. That's the difference.
Both are matters of chance - no one can choose their parents or the circumstances of their birth - but species is only decided once, at the very start. Outside of reincarnation or potent transmutation magic, that isn't something that changes.
By contrast, background is a years-long process that arguably never stops changing, is influenced by innumerable factors, and can be redirected through effort and opportunity, at least in theory.
An orc is always going to grow up to be an orc (again barring reincarnation/transmutation magic).
An urchin might stay an urchin, or they could grow up to be a scholar, or a soldier, or a merchant, or even a noble.