|History as it happens|
Now, look at a person living 500 years ago. That person doesn't view the world by flicking through all these slides and amassing a historical perspective. Their view is constructed as if ALL the previous slides were projected onto a wall at the same time, forming one jumbled 2D image. Almost everything they understand about the world is held in the most recent slides, but there are parts of the slides at the back which show through the layers in front, albeit dimmed.
The person-on-the-spot's view of their cultural/historical context does not suffer from missing slides, because everything that has gone before has some influence on the compound image, but it totally destroys perspective.
There are lots of examples of this in art but I've picked one of my favourites for the top image of this post: Brueghel's Massacre of the Innocents, a depiction of the Bible story about Herod's command that all male babies should be killed because of the Magi's prediction that one of them would become King of the Jews (a position currently filled by Herod). Brueghel depicts the scene exactly in the way of this metaphor: it is a two-dimensional image of an event in which several "understandings" are overlaid to create something that looks strange to us: a snow-covered Flemish town in which soldiers from the 1560's slaughter the inhabitants of a Middle-Eastern village in the year A.D. 1 or so. There is no attempt to depict history as a time-line along which things like dress and technology change, nor to show anything of the differences in location which Brueghel surely would have been aware of.
But the painting is not for the painter, and the intended audience probably did not even really grasp that history existed beyond a fairly simple division of "in my lifetime" and "long ago". We see here a snapshot of how normal people viewed something which was utterly central to their culture: the birth of Christ, and our attempts to reconstruct "true history" without Spanish soldiers or snow or pitched rooves render the image historically ridiculous which in turn hides the fact that this masterpiece is a very precious thing - a view into the historical mindset.
The historian's 3D view smears things out that were never blurred for the people who were there. The fact that Apollo was probably once two deities which became culturally merged is interesting but totally without use when answering "What did followers of Apollo believe in?" Because about all we can be fairly sure of is that they didn't believe that he was two gods.
People can even manipulate these compound views, and Jesus is a good example of this. We know what Christians were told to believe, and almost universally came to believe, after 325's Council of Nicea, but what about in A.D. 225? Well, in 225 plenty of Christians believed that, as a child, Jesus physically abused and tortured the teachers he didn't like at school and was told off by his dad for creating life from the mud around puddles. Historians still know these stories but they have very little impact on the content of Jehovah's Witness leaflets, or any other sect. The Orthodox view stamped out bits that it didn't like; removing them from the projected image that non-historians see.
The dropping of the Atom Bombs on Japan was the final action in a 90-year war for survival which Japan lost, but in that frantic 90 years the Japanese government invented an entire religion (complete with lists of "official" and "obsolete" deities drawn up by the relevant ministry) and a completely new historical context for the nation which the vast majority of Japanese people today take as being truly ancient.
In AD&D we have the price list in the PHB which is great if your campaign is simulating a fairly European 14th century, but there is about as much information about how to modify the list for the Dark Ages or the Roman period as there is in Brueghel's painting, and the same goes double for D&DG.
As DMs we often come to draw up setting based on some real place and/or time and history is one tool we reach for, but it's worth remembering that history is not something that happens to real people, it is just a tool for historians, and real-life culture has no requirement to stick to the truth or even to make sense when dissected from an outside point of view.