When participants first get their DNA results back from the Sinclair DNA study, they are often surprised to find that their DNA matches other families with completely different surnames.
It is important to remember that the use of second names has only been around for about 1000 years. And when they first came out, especially among the Norman people, they were often changed based upon the land one found themselves living on.
For instance I have found records that indicate the family named Villers lived on different land and therefore changed their name. They may in fact be direct brothers of the St. Clair family of England.
Another very interesting name change occurred in about 1164. Willelm de Sancto Claro occurred in the pipe role of 1129 in Dorset in Wiltshire England. He was apparently the successor, and perhaps the son, of Bretel de St Clair in the barony of Stoke Trister, held of Robert Mortain in 1086.
This Willelm was probably the father of Philip de St. Clair and Walter de Ashley who held the barony of Stoke Trister in 1166. There you have a father named St. Clair with a son who had the second name Ashley. This seems to have happened often among the Norman people.
So, if you find a lot of different names appearing in your DNA matches, don't immediately assume that it means one of them is your surname and that you are the product of infidelity somewhere back in time. In the case of the St. Clair family, I believe that the St. Clair's of England were in fact part of a larger group who moved to different land and took many different surnames as a result. Many of these surnames continue to be used to this day, which is why we now match them in our DNA study.