Quite a few of the "matches" ancestry.com has sent for our tree are people with NO family tree, or only a few names on their tree, and most of the time I cannot see any relationship to our tree. I assume that those matches must be based just on the DNA test results, and apparently the DNA matches can go back hundreds of years, way farther than many of us can trace our family trees.
I've yet to get a match to my main family surnames that I'm the most sure about, but that may just be due to a lack of people testing from those family lines.
I've e-mailed to a few of the private tree matches, but I'm finding that most of them don't answer, or don't want to share information - maybe because they CAN see my tree and aren't interested in what they see on it!
So far, of the 15 or so "matches" that I've contacted, two were very helpful and friendly, sharing links and data, four or five just sent a polite response but didn't want to go any farther, one said nothing in my tree matched anything in his, and the others have not replied after a few weeks so I don't expect to hear from them.