More Protestant Christians than not think that Mormons are Christians. But all you have to do is study Mormonism a little bit to see their radically different views. God the father was once a mortal man. God had a god that he worshipped. Jesus was “sired” by god the father. Jesus is literally the brother of Lucifer, as well as you and me. Jesus did not create everything. Our spirits pre-existed this life. We can become gods of our own world. Polygamy is possible in the afterlife. And on and on and on.
Why do so many Christians think Mormons are Christians? Either they don’t know Mormonism or they don’t know Christianity. Or maybe it’s both. So it’s not surprising to see these kinds of statistics. But it does speak of the dire need to train our people.
STR Place Blog 
One of things I discovered when I moved to Utah is that Mormons in Utah are much different then Mormons anywhere else. Many Mormons outside of Utah don’t really even know what it is they claim to believe, so why would it surprise us that Christians don’t see the difference. When one lives near the center of their religion, you quickly discover how different their beliefs truly are, and how their lives reflect that. If one does not understand what Mormons believe they can easily look at them and say “they certainly act like Christians” but fail to understand that their works are dead. Being here you soon realize appearances, effort, and how others view you are central to the practice of their beliefs and that those things are the motivation to their works, not the Holy Spirit. They claim “you will know them by their works” is their proof, yet they are works judged on man’s standards not Gods.
What shocking stats. I think Kelly’s observation that people judge by the world’s idea of what a Christian is but the works of Mormonism are dead. You are right in saying people don’t know Mormonism. So much of it is hidden behind slick presentation. But the real problem you also hghlight is is that they don’t know the Christian faith they claim to believe.