Autograph authenticators aren't particularly reliable...or scrupulous. Both PSA/DNA and JSA have made some really stupid mistakes. At one point a TV station caught JSA authenticating bogus autographs through a sting they did at an autograph show.

http://www.autographalert.com/news.html chronicles some of the exploits of autograph authenticators. It's a good, eye opening read.

As an autograph collector, I believe that you have to become your own expert if you don't want to get ripped off. Get your items signed in person, or pay a little more and get them through sources such as UDA, GTSM, TriStar, Mounted Memories, Steiner, etc. if you want to be sure that they're authentic. If you have to go through some other source, do your homework.