It Only Offers A Probability
You then sequence their genes and look for specific places in their DNA that are the most similar to the group you are testing rather than a different population. With red hair, there are quite a few variants in the same gene that give someone ginger hair.
When it gets more to more complex traits in a person such as taste or exercise, there will be many variants. For this, they only offer a probability of why you have that type of behavior when measured against the population.