How do you derive this definition? And what makes it correct?
Practicality. Way too many cases of self-proclaimed Christians thinking that their own variant is the only "real" Christianity. Most of these can be rationalized, if not justified, by cherry picking the right scriptures. I just accept that they are all Christians, even if they don't accept each other.
Sometimes we see multiple conflicting definitions in play even from the very same people. Consider a man who believes in only an impersonal creator god, but rejects the God of the Bible, and who considers Jesus an admirable philosopher, but rejects the virgin birth, the miracles, the trinity doctrine, and all other claims of Jesus' divinity or supernatural nature. If you ask whether such a man is a Christian, most Christians would say no. Unless, of course, they're arguing that this is a Christian nation and trying to claim Thomas Jefferson as one of their own.