If two women have identical alleles of the suspected 20 height-associated genes, why might one of those women be 5 feet 5 inches tall and the other 5 feet 8 inches tall?

Since there are 20 heigh-associated genes and only two of the alleles are identical to both of them, the other genes could have different effects on the development of their bone and muscle cells which can result to the two women having different heights.