DNA matches - how am I related to these people?!

[kiterunner]

I have a large group of shared DNA matches on ancestry with descendants of a couple called Hans Martin Grider (supposedly born about 1717 in Switzerland) and Catron Brake (supposedly born about 1719 in Germany), who married in 1746 in Virginia, USA. Some of the couple's grandchildren married children of another couple, Rice Medaris (supposedly born about 1745 in Virginia) and Rachel McPheeters (supposedly born about 1757 in Virginia.) I have DNA matches with descendants from those intermarriages, but also with descendants of Rice and Rachel down other lines, and with descendants of Hans Martin and Catron down other lines. Rice's parents are supposed to be John Thomas Medaris and Rachel Davis, both born in Virginia, and Rachel McPheeters' parents are supposed to be Charles McPeters and Mary Ann McDowell, both from Ireland.

Those intermarriages: Charles Medaris married Barbara Grider (daughter of John Grider and Isabel Blair) about 1796 in Kentucky.
James Grider (son of John Grider and Isabel Blair) married Sarah Medaris about 1804 in Kentucky.

So I would have thought this meant that one of my ancestors was descended from one of those Medaris /Grider pairs, but some of the DNA matches in this group are shared matches with someone who is related to me on the Bristow / Lucey lines (their ancestors being Charles Lucey born 1793, brother of my ancestor Ann Jane Lucey, and Elizabeth Catherine Bristow born 1793, sister of my ancestor Robert James Bristow). Which would suggest that the link was further back.

I keep saying "supposedly" because all the information I have about these people is from public trees. But a lot of the info on those particular trees must be right as it connected my DNA matches together.

So, any ideas please? I'm confused!