Answer: GENETICS!!! Here's my proof. Three brothers, same mother, different fathers. Oldest child hair grows extremely fast. Middle child hair grows fast but not as fast as his brother's. Youngest hair grows the slowest. Same manipulation method.
I don't think their hair grows faster/better. I think their minimal manipulation and lackadaisical attitude is what gives the impression that their hair grows faster. Anything you watch over too often will look like it is not making any progress.