Without actually seeing the website it is hard to say. Is the information on the website actual birth certificates or information that people have brought together for display on the web? If it is pulled together from actual sources did they cite the source? If not then it is hard to guess.
If this is fairly modern times and/or the city the name in parens could be the mothers name and Johnsen could be that fathers name. They could have taken "Hope" as a new surname.
Remember, in 1800's people on the farm would be called Jan Olaven (son of Olav) Bruflot (from Bruflot farm).
If a woman moved to the city they could have married a person named Jan Hope she could be called Lizbeth Olavdtr Hope.
A son came from this marriage could be call Elias Jansen Hope and the parens could denote the mother's name (Olesen) or short Olsen.
Again without knowing the web source and the ultimate source, this is only a guess.