I have a little 10 year old girl in one of my families, and I just discovered that her little brother was only 9! What just happened?

My first thought is, is the girl adopted and the boy a biological offspring? It seems that all adoptees are 3 years old at the time of adoption, so if an adoption event occurred when the boy was still 2, and the girl joined the family at age 3, that would make them only a year apart in age. Just my thoughts. Perhaps someone else has another idea.

Both kids are biological, but the younger child was born right after the girl turned 2, so I guess she hadn't had her birthday yet.