I reckon the two records make sense together. The first is from the grave stone, the second from the lair records - so two different sources. The date of death is a little out on one of them - could be either! You may never know which is correct.... (it could be worse, I have a date of death which appears to come after the date of burial for one of mine!)Created by John Haxton in Memory of Christian Hoy his spouse, died 19 Sept 1848, aged 50.
The record from the Fife death index reads
Kirsten Haxton, other name Hay died 20 Sep 1848, buried 23 Sep 1848 aged 51, wife of John Haxton.
The date of burial is the expected few days after the death. She is possibly not in the OPR death records because if you look here:
http://www.scotlandspeoplehub.gov.uk/pd ... 9to459.pdf
...you will see that there appear to be no OPR deaths for the date you are interested in.
Best wishes
Lesley