Why are the readings at Catholic liturgy set up as they are? What relationship does that have to the homily?
 The homily is not so much on the Scriptures as from and through them. In the Roman Catholic tradition, the selection of texts to be read at the Eucharistic liturgy is normally not left to the preacher, but is determined ahead of time and presented in the form of a lectionary. The basic purpose of a lectionary is twofold: to ensure that the Scripture texts appropriate to a feast or season are read at that time, and to provide for a comprehensive reading of the Scriptures. Thus, we find in the lectionary two principles guiding the selection of texts: the thematic principle (readings chosen to correspond to the “theme” of a feast or season), and the lectio continua principle (readings taken in order from a book of the Bible which is being read over a given period of time).
I think I see the distinction the bishops are attempting. If the homily were on Scripture, it would really be more about the teaching and catechesis. Ideally Scripture is the source of content. Ideally Scripture is the means by which God’s love and will are communicated to the faithful.
(All texts from Fulfilled in Your Hearing are copyright © 1982 USCCB. All rights reserved. Used with permission.)