Hi
There a number of reasons, it all depends on the date, there was no formal adoption until, I think 1927ish, a Father could not be named on the BC if the parents were not married, unless both the parents were present, then the BC would be registered in both names, also if the parents married later, I believe that the child's name could then be amended, if you can give us the details, as long as the person is no longer living, we may be able to help further.
Edit Margp