You must have misheard. I said there are three versions of the Ten Commandments. The oldest one is Exodus 34, the second is Exodus 20 and the last is Deuteronomy 5. It is in Exodus 34 that you will find the injunction about "boiling a kid in its mother's milk." This version is almost totally cultic.

If you look again at Exodus 20 and Deuteronomy 5, you will discover that there is not "complete agreement" as you suggest. The primary difference is in the commandment about the Sabbath. Deuteronomy suggests that the Sabbath was to be observed because they had once been slaves in Egypt and even slaves must have a day of rest. In Exodus 20, the original Sabbath Day commandment has been edited to claim that God, resting from the work of creation on the Sabbath, was the reason for its continued observance. That addition to the original fourth commandment was from the quill of the priestly writers in the Babylonian exile (roughly from 596 to 450 BCE, depending on which return from exile was the last one), who also wrote the seven day creation story at the same time. That creation story did not exist when Deuteronomy was written. So the versions of the Ten Commandments are really four: the primitive Exodus 34 version from the "J" writer in the 10th century BCE; the familiar one from Exodus 20, which is originally from the "E" writer in the 9th century BCE but was substantially edited by the "P" writer in the 6th century BCE; and the Deuteronomy 5 version, which is from the 7th century BCE and from the hand of the Deuteronomic writer. The biblical writers accounted for these several versions by suggesting that because Moses broke the tablets, God had to redo them and God did not redo them in the same way.

The fact is that these rules, like all covenant rules, emerged through the life of the nation of Israel and probably always had several versions. That is not a problem unless you are a fundamentalist.