Historians share their knowledge of the past with us knowing that, yes, they can put their own interpretation on events and so on but they have to share the facts so we can decide if that interpretation is right or not. Accurate records are of enormous importance here so who in your fictional world keeps their records? Who decides what the public can know and what should be kept hidden (almost certainly "for the greater good")?
Who decides what the truth is as this can vary depending on who writes the history - the winners or the losers? What historical artefacts/documents are found in your fictional world? Is there the equivalent of archaelogists? Have any "awkward" discoveries been destroyed? Who uncovers the awkward facts and why?
You, as writer, have to decide, of course, what your world is in terms of how it is run, its population and so on, therefore you write the truth. You have to know what is going on to write successfully and bring that world to life. But you don't have to share all of that with your readers. You can deliberately get your characters to keep things back from other characters (to be found out later, almost inevitably). You can reveal bits of your world at a time, let your readers discover Character A is suppressing the truth when for a lot of the story or novel, it had seemed Character A was the reliable "witness". There should be clues that all is not perhaps as it should be with Character A so readers look back and think perhaps they should have guessed. This is particularly true for the crime genre but I think comes into all forms of story.
I have not revealed all that my rebellious fairy godmother, Eileen, knows regarding the Chief Witch, only the fact she does know more than she's letting on. That plants a hint that further revelations may well come later in the story (and one hint can be that Eileen is more than capable of keeping a secret!).
So think about how you handle your world's/character histories and the impact that can have on your overall story? The Lord of the Rings is fantastic here with the background knowledge of how the ring of power came into being. This information adds depth to the story.