Witnesses to an Event

I've found that witnesses on marriage or birth documents provide all kinds of interesting clues and curiosities.

Birth Witnesses (children of James Syme of Northfield and his spouse, Helen) :

James Syme: James Syme of Northfield Esq & Mrs Helen Knox his spouse.College Kirk Parish. A son, Born 24 April 1785 named James. Witnesses--Messrs. James Rainnie, merchant in Leith & James Kettle, Writer in Edinburgh. Bap'd by the Rev. Dr. McFarlane, Canongate.

Christian Syme: James Syme of Northfield Esq & Mrs Helen Knox his spouse.College Kirk Parish. A daughter, Born 28th July 1786 named Christian. Bap'd by the Rev. Dr. McFarlane, Canongate. Witnesses--Messrs. Archibald Knox of South Drum & James Kettle, Writer in Edinburgh

George Syme: James Syme of Northfield Esq & Mrs Helen Knox his spouse.College Kirk Parish. A son born 21st Oct last [1788] named George. Witnesses-- James Rannie, Jr. Merchant of Leith & James Kettle Writer in Edinburgh. Bap'd by Rev. Mr. Robert Walker, Minister Canongate.

Andrew Syme: James Syme of Northfield Esq & Mrs Helen Knox his spouse.College Church Parish. A son, Born 9th Oct. last [1789] named Andrew. Witnesses--George Knox, Esq and James Kettle, Writer.

Thomas Syme: James Syme Esq of Northfield & Mrs Helen Knox his spouse. College Church Parish. A son Born 5 Nov, 1795 named Thomas. Witnesses-- Messrs. George Knox and James Kettle , writer in Edinburgh.

David Syme: James Syme of Northfield Esq & Mrs Helen Knox his spouse. College Kirk (?) Parish. A son Born 29, July 1797 named David. Witnesses--George Knox and James Kettle. W__ Edin__.

Catharine Syme, the youngest, seems to be nowhere to be found in the birth records although all the other children seem to be named and recorded. That has always made me wonder about her.

Another curiosity is the spacing between Andrew and Thomas. The other children seem to be born within a year or two of each other but between the said two boys there is about a 5 year gap. I wonder if there were miscarriages, stillbirths. Even a separation or illness of a parent may have induced such a spacing.