Hi Averil,
I think you are getting the Robert Christians mixed up (or I am). I understood Bob to mean that there were two - father and son. The son was Bob's ancestor who emigrated to NZ and married twice in NZ giving his place of birth as Isle of Man. Bob gave Robert's parents (from certs?) as being Robert Christian and Margaret Anne Boyd who presumably lived in the IoM. - So the mother Margaret Anne Boyd wouldn't have been on the ship or in NZ.
Sue