In their different readings of Torah this Shabbat, both Orthodox and Reform traditions make serious statements about God's nature and how it is reflected in the Exodus. For the Orthodox, who read Exodus 34:6, there is a direct connection between the Thirteen Attributes of God and this Passover season. God has certain qualities. Out of those qualities came the Exodus. We therefore praise God.
Reform are less sanguine about the exact nature of the hand of God in history. They read Deuteronomy 10:17, which tells of God's nature but are more general in their choice of Torah's description of God's qualities and the way that God acts. At this Passover season we know God was somehow behind the Exodus. How? We are not so clear.

Of course, Reform have the reticence of Jewish tradition on which to build this attitude. Think of the v'hayah im shamoa paragraph of The Shema and its blessings: "And it will come to pass if you will hearken": why is it said in silence? One view is that the rabbis were disturbed by the mechanistic nature of this paragraph. They knew that devotion ("hearkening") is not always followed by reward (God's bounty). So, it is said, rabbis ordained: say it in silence and it will not be so embarrassing.

Jewish tradition often edits the past because tastes change. Think of Exodus 34:7. The Torah has v'nakeh lo y'nakeh, "He will by no means clear the guilty", while the prayer book has "forgiving transgression and sin" (Singer). Tastes changed: the Torah could permit the possibility of some sins not being forgiven. The Prayer Book authors could not live with such a dreadful possibility. Some could reconcile the change from Torah to Siddur. Many just live with difference.