As with pretty much everything, I think it's one's genetic endowment being mediated by environmental factors. Your genes contain the instructions for you to grow, but things such as chemical exposure, stress, nutrition, etc., will affect the extent to which you would reach this genetically predetermined height. I might just be regurgitating bullshit, but I'm sure there's a significant height difference between North and South Koreans that's thought to be down to a lack of nutrition.
Edit: I didn't see the geneticist's comment in the photo, but yeah, I think he nailed it. Genes, environment, and whores, lol.
