With respect to the following record:
Marriage, David REEKIE (father Michael REEKIE) and Elizabeth BELL (father David BELL), 18 March 1851.
This might be the first proclamation of the banns - the actual marriage might not have taken place until after the date of the census (30 March 1851).
This could explain why David, aged 21, was still at home with his parents when the census was taken ... But it would be necessary to view the actual image to see whether he is listed as married or unmarried.
There is a possible Elizabeth BELL in 1851 - though rather young
Main Street, Leslie, Fife
BELL
David 56, Head, Ag Lab, b Longforgan Perth
Mary 54, Wife, Flax Winder, b Kennaway Fife
David 24, Son, Ag Lab, b Seven (probably should be Leven), Fife
Mary 21, Daughter, Bleacher, b Leslie Fife
Elizabeth 15, Daughter, Hand Loom Weaver, b Kinglassie Fife
Then by 1861, Elizabeth has probably passed away. Perhaps in childbirth with Elizabeth born 30 Nov 1860.
JAP