Records at www.mullgenealogy.co.uk show that the wife of John McKinnon, born about 1767 and died 30 December 1855 at Fanmore, Isle of Mull, was Mary McLean.
However the death certificate of their son Lachlan MacKinnon states that John MacKinnon's wife was Mary McDonald. This information was reported by Lachlan's son, Lachlan the younger, and may have been a little confused if the younger Lachlan was distressed when reporting the death information to the registrar.