Why do some people have blue eyes, others brown and still others something intermediate like grey or hazel? It's just genetic variation, probably the result of long ago mutations.
The same is true for blood types, left and right handedness and a whole lot of other variations. For the sake of those who worry, these variations are controlled by pairs of genes and some genes are dominant and some recessive. For example a person with group A blood may be AA or AO. Similarly, for Group B the person may be BB or BO. If they have a child he/she could be AO (group A), BO (group B), OO (group O) or AB (group AB). It is perfectly possible for parents who are both rh+ to have a rh- child.
What is the reason for genetic variability? Better survival value for a population because if something evolves that attacks all the people with Group A blood, then the people with Group O and Group B would still survive.