Seems I've stumped everyone

It is possible that Katharine's maiden surname was also Mckenzie. I have come to this assumption
going on the register with Murdoch and Katharine's son John's birth. Murdoch and Katharine are both accorded the same surname, McKenzie, however other births listed on the same page have different (ie maiden?) surnames for husband and wife.
Is this theory plausible??
I have found John Mckenzie and his wife listed in the 1881 census in Boathaven, Wick. In the 1891 census
Isabella is listed as a widow however I can find no record of John's death

Was hoping to find this with his parents names on it.
Any takers??
Bec