On Alexander's marriage certificate in Urquhart, Morayshire on 18th December 1856 it gives parents as Alexander Mckenzie and Jane Kerr. On his death certificate of 2nd January 1913 in Urquhart, Morayshire it gives them as Alexander Mckenzie and Jane Carr. I have also picked these up from IGI. Up until the marriage and death I am sure as I can be that I have the correct Alexander.
It's quite common to find Kerr and Carr used interchangeably. So a Jean Ker(r) and a Jane Car(r) are quite likely to be one and the same person. There is no definitive way of spelling it.
I have an Alexander Mckenzie having married a Jean Car on 16th October 1824 at Fordyce. This was extracted from GRO (no image). Also Alexander marrying a Jane Ker on 2nd November 1824 at Rathven.
Both entries are 'extracted' from the original parish registers. The spelling is of no significance - it simply reflects how the parish clerk thought the names should be spelled.
What it tells you is that one of the couple lived in Fordyce and one in Rathven when the banns were called. The banns had to be called in both parishes of residence. The original entries will tell you which one lived in which parish.
The difference in dates isn't significant - one parish clerk may have written the entry on the day the banns were first called, and the other may have added the actual date of the marriage. Again, the original entries will clarify this.