good question. i think this comes from the fact that many bible verse contradicts itself if you just pit two verses against each other.
for example:
1 John 3:8
He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
1 John 1:8
If we say that we have no sin, we deceive ourselves, and the truth is not in us.
these are in the same book. no author is foolish enough to just write two contradicting statements like this in the same book. so what comes from this is that people dont harmonize the bible with the totality of it. and you have one side of the people quoting the verse below to excuse their living in sin and total rebellion against God, and you have the other side quoting the verse above preaching sinless perfection and if you sin you just aren't saved, no one is saved (except for they of course)
this is how these problems arise. another similar situation:
Romans 2:13
For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified.
Romans 3:20
Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
here we are again. one side posting the other verse etc.... this contradiction can easily be solved by reading the context, without chapter breaks its clear that what st.paul says in Romans 2:13 is true, if you do the law of God you will be justified, but if you keep reading you notice the problem is that both jews and gentiles have not kept the law and they both need God's grace to forgive them their sins and that is why in actuality, by deeds of the law no one will be justified.
Romans 3:9-10
What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin;
As it is written: “There is none righteous, no, not one;
hope this answers some questions. i know these are just couple verses, but this is how different demonations and interpretations come.