The 1871 census includes son Robert, aged 9, birthplace Norway. It turns out that daughter Mary Jane, who was born in Leeds in 1857, also has an overseas birth certificate, issued in Christiania (now Oslo) in 1860. So I hypothesize that the family set sail for Norway with a new-born daughter for whom they had not had time (or bothered) to register the birth. Then, perhaps when it came to return home, they realized that they needed to register the birth.
Interestingly, this Mary Jane and her older sister, Sarah, eventualy married a pair of brothers, one of whom was my great-grandfather!
Tim