Why does the Catechism of the Catholic Church abridge the first commandment? (This question came up while I was speaking with someone, and I don't think that I was able to answer it very well. Note that I am not asking whether the Church is right or wrong, just for a clear explanation of why Catholics have chosen to present a particular document in a particular way.)

In part 3, section 2, the Catechism of the Catholic Church lists the Ten Commandments. It provides a table with three columns: "Exodus", "Deuteronomy", and "A Traditional Catechetical Formula". 

Why is the First Commandment abridged in the third column? Why aren't the commandments numbered in the first two columns? Is this meant to be a table of revisions, with the right side representing the "revised" commandments? What is the third column even supposed to be? Which of the columns is the "official list" of the 10 things Catholics are supposed to obey? Is the Catechism changing scripture?
The answer, I think, is straight out of the "training manual" for filtering and interpreting the Catechism:


  Inculturation of the faith, under certain aspects, is a linguistic task. This implies that catechesis respect and value the language proper to the message, especially biblical language, as well as the historical-traditional language of the Church (creed, liturgy) and doctrinal language (dogmatic formulations). It is also necessary for catechesis to enter into dialogue with forms and terms proper to the culture of those to whom it is addressed. Finally, catechesis must stimulate new expressions of the Gospel in the culture in which it has been planted. In the process of eculturating the Gospel, catechesis should not be afraid to use traditional formulae and the technical language of the faith, but it must express its meaning and demonstrate its existential importance. Similarly, it is also the duty of catechesis "to speak a language suited to today's children and young people in general and to other categories of people—the language of students, intellectuals and scientists; the language of the illiterate or of people of simple culture; the language of the handicapped, and so on".


#208 - General Directory for Catechesis

Basically, the Catechism of the Catholic Church is supposed to be a template for all particular Catechisms to follow. It's the official interpretation of scripture.  Not the only interpretation, but certainly the most broadly acceptable


  In virtue of being a catechism, the Catechism of the Catholic Church collects all that is fundamental and common to the Christian life without "presenting as doctrines of the faith special interpretations which are only private opinions or the views of some theological school" 


#124 ibid.

So, if you're looking for the Catechism to pick a fight with the Bible, you've came to the wrong place.  In the instance of the Decalogue, the Catechism lists the broad statements that justify each of the commandments as subheads to subsequent sections. Which, from the standpoint of being good literature, makes perfect sense.