As I remember, Marie Curie's husband took her maiden name when they married. You don't mention where or when your great grandfather married.
In some societies , and at some times in the past this happened. Also, in some areas, if the wife belonged to a famous family, the husband may or may not take her name, but the children would carry the mother's maiden name as a last name.
You need to look at the place and time when that happened and see if others were doing that and why.
On the other hand, it could be a clerical error in a particular document. If that is the case, then his own surname would show in other documents, such as on a death certificate.