I’m occasionally reminded when going through my AncestryDNA matches that shared matches are helpful, but that they can be misleading as well.
My match Linda shares 28 cM with me on 1 segment. She is a descendant of my 3rd great-grandparents Clark and Mary (Dingman) Sargent. The shared match with whom I share the most DNA myself is one I’ll call “cc.” Cc is a known descendant of a different set of 3rd great-grandparents, Mimke and Antje (Jaspers) Habben. The Dingmans and the Sargents have no connections to each other. The Habbens were Ostfriesens and the Sargents have no known connections to that area at all.
The other shared matches with Linda are all other known descendants of ancestors of Clark and Mary (Dingman) Sargent. Cc is an outlier in terms out analyzing my matches. Somewhere in their ancestry, cc and Linda have a common ancestor with each other that they do not share with me.
Do not assume that all the shared matches you have with a person have to be related to you through the same family. It is possible that they share a connection with each other that they do not share with you.
This is just one reason why it is important to identify as many of your matches as you can…even on those families “you didn’t test DNA for.”