I have been called a Christian several times in the past few days. I usually don’t argue about that because I am at the very least a “cultural Christian.” If I’m arguing with an atheist it’s not worth the trouble to correct him that I’m a Mormon not a Christian. But I make the distinction with friends and family.
One of the reasons I call myself a Mormon is that the LDS Church insists that I don’t. It works very hard to paint itself as just another denomination of Christianity. I think that is a mistake. The official line is that the Book of Mormon is a complement to the Bible. That is a Christian position and one who accepts it will feel comfortable calling themself a Christian. I do not accept that position. I think that the Bible is a historical context for modern day revelation and that the Book of Mormon is superior to the Bible. I differentiate my position by calling myself a Mormon.
But there is a deeper issue. I was raised very close to Christian orthodoxy. I continue to work to escape it. Traditional Christianity seems very backwards and foreign to me. I reject the Trinity much more significantly than LDS doctrine does. It seems much more likely that there are three (or more) gods than a single three part god. That is obviously a Mormon position not a Christian one. So if you have read this far and you still care, that is why I call myself a Mormon.