St. Thomas on Interpretation of Scripture

The author of Holy Writ is God, in whose power it is to signify His meaning, not by words only (as man also can do), but also by things themselves. So, whereas in every other science things are signified by words, this science has the property, that the things signified by the words have themselves also a signification. Therefore that first signification whereby words signify things belongs to the first sense, the historical or literal. That signification whereby things signified by words have themselves also a signification is called the spiritual sense, which is based on the literal, and presupposes it. Now this spiritual sense has a threefold division. For as the Apostle says (Hebrews 10:1) the Old Law is a figure of the New Law, and Dionysius says (Coel. Hier. i) "the New Law itself is a figure of future glory." Again, in the New Law, whatever our Head has done is a type of what we ought to do. Therefore, so far as the things of the Old Law signify the things of the New Law, there is the allegorical sense; so far as the things done in Christ, or so far as the things which signify Christ, are types of what we ought to do, there is the moral sense. But so far as they signify what relates to eternal glory, there is the anagogical sense. Since the literal sense is that which the author intends, and since the author of Holy Writ is God, Who by one act comprehends all things by His intellect, it is not unfitting, as Augustine says (Confess. xii), if, even according to the literal sense, one word in Holy Writ should have several senses (ST I Q1 A10).

Compare this with the Catechism:

According to an ancient tradition, one can distinguish between two senses of Scripture: the literal and the spiritual, the latter being subdivided into the allegorical, moral and anagogical senses. the profound concordance of the four senses guarantees all its richness to the living reading of Scripture in the Church.

The literal sense is the meaning conveyed by the words of Scripture and discovered by exegesis, following the rules of sound interpretation: "All other senses of Sacred Scripture are based on the literal."

The spiritual sense. Thanks to the unity of God's plan, not only the text of Scripture but also the realities and events about which it speaks can be signs.
1. the allegorical sense. We can acquire a more profound understanding of events by recognizing their significance in Christ; thus the crossing of the Red Sea is a sign or type of Christ's victory and also of Christian Baptism.
2. the moral sense. the events reported in Scripture ought to lead us to act justly. As St. Paul says, they were written "for our instruction".
3. the anagogical sense (Greek: anagoge, "leading"). We can view realities and events in terms of their eternal significance, leading us toward our true homeland: thus the Church on earth is a sign of the heavenly Jerusalem

(CCC 115-117)

Because God is the author of Scripture, the things signified by the words of the Bible also signify things. On this basis the Church explains the different senses that we must be ready to find in God's Word. The literal sense is the basis for the others, but it is not the only one. It would be absurd to presume that the only sense of Scripture is that which was intended by the human authors of the Bible.

St. Thomas goes on to say:

The multiplicity of these senses does not produce equivocation or any other kind of multiplicity, seeing that these senses are not multiplied because one word signifies several things, but because the things signified by the words can be themselves types of other things (ibid., ad 1).

For a fascinating example of the use of the Bible by St. Thomas, see his exposition of the Hail Mary.