First, we should distinguish between "Genesis" and the "Creation Story." It's believed there are several different authors to Genesis written (and refined?) at different time periods because of a difference in writing style. Focusing in on the creation account, the imagery and systematic unfolding of the story makes the account less literal than, say, the gospels. Even some of the events are very difficult to understand if one takes a strictly literal reading of the text (e.g. Adam being created, having enough time to feel lonely, name all the animals in God's creation only to find no suitable helper, be put to sleep, God performing surgery, Adam waking up, and God bringing Eve to him all within a literal 24-hour time span).
I'm sure a more literary specialized person could tell you more, but the styles are certainly different. The gospels and epistles have a difficult time making any sense at all if not taken as a literal, historical account. The same just isn't true for the creation account in Genesis.