My sister hair grows 2x as fast as mines.... although she doesn't relax she does dye her hair, blow dries, flat irons, uses grease with mineral oil..she loves pink lotion....and her hair grows like weeds.
I am natural and have been for over nine years.....I haven't done half as much as my sister has done to her hair -- and even when I started taking care of my hair...her hair still grows faster.
I think I am one of those ppl that only gets 3 inches a year. It really does depend on the person..... and even in families it varies.....