The problem that continually arises when trying to properly interpret a culture's mythology is the definition of the word "myth."
As used by Joseph Campbell, Mircea Eliade, Marie-Louise von Franz and others, a culture's myths are the stories that are told containing cultural archetypes, heroic figures, epic confrontations and/or magical occurrences. This approach to the study of myth assumes that the various aspects of mythology represent externalizations of internal, psychological processes in humans and by studying myths as archetypal examples we can better understand the stories of our own lives and the assumptions we make about ourselves and our interactions with other people.
The term "myth" for Claude Levi-Strauss has a different meaning. Myths are stories because, in an oral culture, storytelling is the means by which cultural information is transmitted from one person to another and across generations. The mythic heroes and monsters, magical activities and impossible events are memorable because they need to be to preserve the information being transmitted. With our modern sensibilities, we look at the absurdities and inconsistencies of fairy tales and myths and assume we have discovered evidence that "primitive" thought processes are illogical and immature when compared to modern thought.
In his series "Mythologiques", Levi-Strauss has suggested that myths are not important because they present archetypal images; myths are important because they demonstrate an externalization of human thought processes. Levi-Strauss assumes that since we are all members of the same species, that the thought processes of less technologically advanced peoples are the same as our own, just applied to different objects. By interpreting the logic of myths, we can understand how the mind works.
It is also assumed that individual myths that have been passed down to us may be incomplete. In order to understand the "message" we must contrast and compare multiple variants of the same tale. The true message of a myth is revealed when one is familiar with its place in the total cultural context that generated it. That's why saying that the Superman myth, for example, is the basis for all of our culture's archetypes is misleading. Superman(1948) is not Superman(1960) or even Superman(2007). If you doubt this, I direct your attention to the site http://www.superdickery.com/ for an alternative interpretation of the Superman myth.
The reason I focus in on advertising is because of the vast body of examples advertising supplies. Advertisements are constantly changing, constantly reflecting current cultural conditions, and self-validating through sales trends.
It may be possible to arrive at valid conclusions concerning our culture using just the five Harry Potter movies or the Superman series. However, the possibility for individual influence on the "message" is far greater with artistic control centralized in one person or a small group than in a much broader body of work.
Advertisements work over space the way myths work over time. Mythic stories survive over time because they resonate with the population. An individual advertisement may have a much shorter shelf life, but because it has to be distributed throughout a large population, it must also resonate to be effective.