As was the case at that time many children were born. David Boyd had 2 wives. The first Margaret Blackwood who is Henry's mother. She died around 1800 and David remarried Agnes McNeilly. She was much younger than David. They had 7 children that I know of and the youngest was William My GG Grandfather was born 1821.
In my own family, my mother and my older sister both gave birth a few days apart in the same year.
In big families this happened.