Just because something is in a document doesn't mean that it is reliable. These documents have been written by people who are biased, who may take on a tone which causes the reader to be biased and not have a balanced view on what really occurred.

Also, those who have written the document may not have the whole story, or information needed to have a complete overview of an event. Especially with Ancient history, the sources are limited and so we can only rely on ancient historians who have gathered their information through the testimony of others who had been witness to the event, or heard from someone else who had been witness to the event. Not exactly reliable.

Just because something is written does not mean that it is true.

One example is Roman history which was heavily biased. This can be seen in the actions of Octavian (Augustus Caesar) who used propaganda to put down one of his opponents - Sextus Pompeius - whilst discussing the other - Marc Antony - at great length. This resulted in a very poor image of Sextus, barely any mention of him in historical references, being accused of being a pirate and being cowardly, whilst Marc Antony was hailed.

It's called censorship and politics.

Nothing is truly reliable, not memories, not the documents that memories are made from.