The same way there are differences of colour and hue. It is seen that babies of different colour and hue take birth from the same parents under the same circumstances. All this is the marvel of Allah's creation. The wisdom behind the difference of colours requires a lengthy description which is beyond our scope, but many of these wisdoms can be easily understood by a little reflection.
While referring to this sign of the divine omnipotence, this verse Ar-Rum 30:22 ⧉ has mentioned creation or many things, such as sky, earth, the difference of languages and dialects, difference of colour in humans, which are all signs of Divine wisdom and can be recognized and understood with little attention. Hence, it is added at the end of the verse:إِنَّ فِي ذَٰلِكَ لَآيَاتٍ لِّلْعَالِمِينَ (Surely in this there are signs for the persons of knowledge - Ar-Rum 30:22 ⧉).