Simple (maybe overly simple, but simple) answer: if the Bible "contains" the word of God, then there are parts of the Bible that made it in, not on the basis of God's will, but human cultural additions. To put it even more simply, if the Bible "contains" the word of God, you don't have to worry about all those silly anti-homosexual-act references, or how divorce is a really bad idea, or wives submitting to their husbands (or husbands having to love their wives sacrificially), or how God loves Israel and will whoopa$$ anyone who attacks it, etc., etc.
You are right. And, because we know that ALL Scripture is God-breathed, we can discern that those extra-canonical books (Apocrypha/Deuterocanonical) that some claim to belong along side the rest of the universally recognized writings are NOT Scripture. Of course, some want to make a big issue over these books and imply that "they" have the whole Bible unlike those of us who are missing out and, somehow, this proves their claim to be the one, true church. But, we know that God would never make mistakes - of which those extra books contain many such errors - and He would never contradict Himself. The prophets of old whom God used to reveal His sacred word KNEW they were speaking as they were carried along by the Holy Spirit. You will find no such assurance from the writers of those seven books - if they are even known (some leave open the identity of the author). That is how we CAN know that the 66 book canon is the correct one and, as such, we CAN have assurance that what God tells us is the truth.
Or having to kill witches, or having multiple wives, or slaves, or not eating bacon, or killing your child for denying god, etc., etc.
The point being, that even the most ardent, "the Bible is the word of god" posters here, pick and choose what parts they accept. Thank god.