As has already been pointed out, there were at least two Catherine Kerr Steed/Steads of similar age.
You say you have a birth certificate of one of them, but how do you know she is the right one?
I see a marriage of a John Steed to Catherine Kerr in Dalmellington in 1872.
They had a son James, born in 1874 in Cambusnethan, who is an exact match for the 26-year-old James Stead in the 1901 census.
They also had a son John, born 1878/9, who could have been 21 on 7 May 1900 when John Steed and Agnes McMurran were married.
So your two Catherine Kerr Steed/Steads are almost certainly first cousins, both named after their grandmother, and you need to sort out which cousin was the wife of William Samson.
Noting that William Samson and Catherine Kerr Stead/Steed named their first daughter Elizabeth does rather suggest that William Samson's wife was the daughter of James Steed/Stead and Elizabeth Owens rather than of John Stead/Steed and Agnes McMurran.
As does the evidence of the deaths indexes which has already been quoted.
And forget about Ancestry. Go for the original documents on Scotland's People just to make sure that what is on Ancestry wasn't put there by someone who hadn't bothered to do the basic research properly.
