At the physical level, truth is absolute. The tree that fell in the forest, unheard and unseen, still lies on the ground, philosophical speculation notwithstanding. But the account of human affairs that we call History, and that we make the subject of college courses, has little to do with truth. It is information that our rulers want us to have. It is information that allows us to understand how our world, our way of life, is ordered. It is the data on which the social compact is based. But the social compact is not a done deal; it is negotiated and re-negotiated over time. Think of a labor contract, and how that changes over time.
As the terms of the agreement between workers and managers, between laborers and owners, change, so the story that they tell to explain the changes must itself change. Child labor, for example, was not always an evil thing. In the same way, as the social compact is rewritten, the conditions of life changes and with that, our view of ourselves. These changes require concomitant changes in the stories we tell or ourselves. These stories are our history.