Excellent analysis @gavvet, I've often pondered this myself. In addition to the significance of the numbers, it seems that explaining creation broken down into days would have been the best way to convey it to a society that did not have an understanding of some of the more complex concepts that a deeper explanation would have involved. In the same way, I don't think that creation excludes the possibility of evolution (within certain boundaries) and the world being millions (rather than thousands) of years old.