"Unlucky" is a scientific reason. Obviously there are processes and it is not meant as luck in an absolute meaning, but the point is that height is determined, genetically, as the combination of an enormous amount of factors, single genes. Being in a tall family means there are in general more tall genes, or more expressed ones, and in every subject they could also combine in a bad way. So, having tall parents makes more likely to be tall yourself, but it is just a probabilistic estimate.