It could be that one is going by the mother's maiden name and one is going by the father's surname. I work with brothers:
Marcos Peòa
Alejandro Peòa, and
Julio Suarez. For whatever reason, I haven't asked yet,
Julio uses his mother's maiden name. Technically the brothers' last name is Peòa-Suarez.
Julio is the youngest brother,
Alejandro is the oldest.