A given prophecy will be internally consistent, but not externally so, beyond following the rules laid out here. For example, the Six of Dragons represented Decius to Raitlor; in a different prophecy, it could represent a skilled business competitor, or an actual dragon. But it will never change meaning during a given prophecy.
The seven cards exclusively represents the gods, or the Godslayer. Which god is represented by which card can change from prophecy to prophecy; Kilmorph and Mulcarn both fit under the Tower. However, the Sword, Pentacle, Sun, Moon, Cup, Tower, and Wand are usually associated with Junil, Agares, Lugus, Esus, Danalin, Kilmorph, and Cernunnos, respectively. While other gods can be represented by these cards, if any of the aforementioned gods are involved, they will be represented by the Seven of their suit, with priority over all other gods and associations.
All other numbers are scaled according to the prophecy's scale; a particularly fierce local businessman who is unimportant outside of the village he controls with an iron fist could be represented by a Six (if the prophecy is about a villager's life) or a Three (if it is about the county, or possibly even the kingdom).
Each suit represents the following, more or less:
Angels: Holiness, literal angels, and to a lesser extent, kindness and mercy. (For instance, the Elohim would usually be represented by the Angels; however, they'd be moved to the Sun or some other suit if the Mercurians were also involved.)
Demons: Unholiness, literal demons (unless Agares is directly involved), and to a lesser extent, cruelty and depravity.
Dragons: Action, ambition, literal dragons, and to a lesser extent, rage. (A dragon will sometimes be associated with the six of its god's suit, however; Abashi would be the Six of Demons if Ceridwen was the Seven, and Eurabatres the Six of Angels if Amathon was the Seven of Angels. This frequently, but not always, leads to the dragon's slayer being represented by the dragon suit.)
Sword: Literal items of power (including swords), and to a lesser extent, military force and order (although military force is more intimately tied with the Dragon; if there are two forces clashing, the more organized will be Swords, the less organized Dragons).
Pentacles: Arcane knowledge and power, and to a lesser extent, mystic symbolism. (Unless Agares himself was involved in her prophecy, the Prophetess would represent herself as a Three of Pentacles; she does not consider herself important enough to warrant any higher number.)
Sun: Revelation, philosophical knowledge, and to a lesser extent, fire and light.
Moon: Passive mystery, active deception, and to a lesser extent, the night, darkness, and death. (If Agares was involved in her prophecy, the prophetess would represent herself as a Three of Moons.)
Cup: Freedom, change, and to a lesser extent, water, ships and literal cups.
Tower: Stoicism, conservatism, and to a lesser extent, earth and literal constructions.
Wands: Nature, balance, and self-knowledge. (And, once more: I should have represented Laroth as a Six of Pentacles; I hadn't thought about the question of exact representations well enough, though.)
The Fool: Whoever is being told the prophecy.
Death: The One, or Armageddon. The prophetess cannot differentiate between the two, as both bring her visions to an abrupt end, but she fear the unknown and assumes both are malevolent forces to be opposed.