The eminent Swiss psychiatrist, Dr. Carl G. Jung noticed that it's human nature for us to give into the temptation of "flattening" out (thus killing) the vibrant, complex, multiple meanings of symbols. We give into this temptation in order to codify the meanings of symbols into a comprehensible set of formulae that we can then establish as a set of laws and rules to follow. Jung's single biggest "bone to pick" with astrology (and astrologers) was this same natural human tendency to flatten out, oversimplify, and thus kill the symbols of astrology. i.e. Mercury means nothing more than communication. Jupiter means nothing more than expansion. Venus means nothing more than relationship

The Sun As Symbol
"An archetypal content expresses itself, first and foremost, in metaphors. If such a content should speak of the Sun and then identify it with the lion, the king, the hoard of gold guarded by the dragon, or the power that makes for the life and health of man... It is neither the one thing nor the other... but rather the unknown third thing that finds more or less adequate expression in all these similes, yet - to the perpetual vexation of the intellect - remains unknown and not to be fitted into a formula." Short Excerpt from: The Archetypes and the Collective Unconscious C.G. Jung

Jung reminds us here that the images typically associated with the Sun's heart energies are the lion, the king, the hoard of gold guarded by the dragon, and/or the power that makes for the life and health of man... The Sun is all this, and yet it is so much more! However, as Jung pointed out, the Sun (as well as, all other archetypal images and symbols) - "to the perpetual vexation of the intellect - remains unknown and not to be fitted into a formula." The very moment we attempt the flattening out of the Sun's symbolic meanings for our own purposes - by declaring with surety that the Sun is this one single thing - it suddenly confounds us by becoming yet another unknown third thing.

Hermes and Zeus

Back to the Source
Taking a closer look at Greek mythology sends us straight back to the original source of the where's, why's and who's of the traditional meanings behind the symbols used in astrology. Using Greek mythology, in the study of symbols, can help amplify the various meanings behind a symbol.

For example, through Greek mythology, we can better understand why Mercury (Hermes), the planetary ruler of Gemini, eventually ended up being associated with short distance journeys and why Jupiter (Zeus), the planetary ruler of Sagittarius, became associated with long distance travel. To illustrate this, here's a quote from mythologist Karl Kerenyi's book entitled Hermes, Guide of Souls.

Speaking first of the traveler:

"His (the traveler's) guardian is not Hermes, but Zeus, the god of the widest horizon and the firmest ground. In contrast, the situation of the journeyer is defined by movement, fluctuation. To someone more deeply rooted, even to the traveler, he (the journeyer) appears to be always in flight. In reality, he makes himself vanish ('volatilizes himself') to everyone, also to himself... With companions of the journey, one experiences openness to the extent of purest nakedness, as though he who is on the journey has left behind every stitch of clothing or covering."

Traveler vs. Journeyer
Here, Kerenyi distinguishes between the traveler who is ruled by Zeus (Jupiter) and the journeyer who is ruled by Hermes (Mercury). For the traveler, the trip is merely a way to get from "here" to "there" (from point A to point B). Zeus (Jupiter) is the guardian of the traveler who is going long distances. For the journeyer, the trip and the adventures experienced along the way are what matter most. And Hermes (Mercury) is the guardian of the journeyer.

Different Versions
As for why are there different stories, strains and versions of Greek mythology? There just are... The ancient Greeks didn't seem to be hung up on needing to have only one version of a story or explanation. As for learning more about mythology, one of the single biggest tips I can pass your way to not to simply rely on books written by astrologers to learn about mythology. We have a tendency to be too inbred as it is.