I can't remember if this was mentioned, but when you find these people who match are they actually descended from the person who took the DNA test or are they just in their tree? There are people here with huge trees that include many people (in some cases most people) who are not related to the DNA test taker other than by marriage. I'm also not sure exactly what the new surname search is looking at. If it looks at everyone in the tree then that could be an issue.
Instead of using ancestry's new surname search, I suggest you using Jeff's chrome extension. I used that to get a count of exactly how many users/tests were in common with the 4 tests that I manage. Based on that I haven't seen the type of thing you see. My wife and mother are from totally different backgrounds, and there is no overlap other than a few very low confidence ones which probably aren't real.