How can a baby be born with a different blood type than the parents?

Happens all the time: That's normal and happens all the time. Some combinations are not possible, such as 2 a biological parents having a b baby.

Genetics: The parents have blood types that are made up of two alleles. That is to say they have a blood type made up of a, b or o. Each parents have two of these alleles. Mom could be aa, ab, ao bb, bo or oo, and so could dad. Once these match up they give the baby its blood type. Mom is oo and dad is ab, then baby could be either bo or ao, that will be reported as type a or b. Hope this helps.