Well, then how do you explain all the doctrinal differences amongst groups that you DO accept as Christian?
That's like comparing apples to oranges now. They're both fruit....and they both agree on the major issues as concerns what they are (the Trinity, being one); but they have a different understanding as to how to "best please" God. And seeing as how God didn't make us robots or clones, rather, individuals with a free will, it's rather obvious, isn't it, as to why there would be difference in how to perform the minutia concerning how to 'best' worship Him.
It's sort of like macro vs. micro Economics. They agree on the macro (really important stuff....that would be concerning the nature of God and salvation through Christ's blood on the Cross), they don't agree on the micro part (the part that isn't necessary for salvation). I'm less concerned about their differences over the micro than I am concerned about the differences in the MACRO: Mormonism v. Christianity. They ain't the same at all.