Maybe its because they haven't actually read the Bible, they just rely on what their preacher/"father"/whatever "teaches" when they attend their church, which is what most of the Christians in the world do. Of course since said Preacher will never preach to their flock about certain quite amoral passages, they don't know their God is a bloodthirsty mess of self-contradiction who encourages ignorance, homophobia, incest, hate, slavery and etc.

If they actually know the Bible, then they most probably dismiss/ignore the bad parts. If we are lucky they acknowledge the Bible is just a human-written mess and they only retain the overall modern "Jesus is love" approach and believe in a deity (like "something superior" or the "energy of whatever"), which is just their personal appreciation if the god the bible depicts, they are more of deists with a Christian dent than actually Christians. If we are less than lucky they belong to that particular group of people who will shut their ears when you point the inconsistencies of their belief system. There's a thing but I cant recall the name (I'm sure it was some form of dissociation) where you can hold contradicting beliefs or for example you hate black people, but you have dissociated that your black best friend is, well black. Anyone remember what this is called?