Ancestry reports the Most Likely Estimate (MLE), which is not of much value for research. See this post:

http://boards.rootsweb.com/topics.dnaresearch.ydna/54.1/mb.a...

There may have been a non-paternal event (NPE), which is usually defined as a son's or unmarried daughter's use of a surname other than that of his/her biological father in countries which have fixed surnames. Even with a 46/46 match the event could have occurred over a wide range. We have one that occurred 225 years ago with a 46/46 match.