The variances in predicted vital capacity as related to height has to do with the size of the lungs and amount of room for the diaphragm to expand inside the body. Height is only one factor that is calculated into the vital capacity measurements.
Age, sex, height, weight and even race all play a factor into what a person's vital capacity is. These values are normally compared to already complied lists that the techs who administer the tests have access, too. This shows what a person of a certain set of factors should approximately have as test results. Since some minorities can differ from this information, their results may show up abnormal.