You have to consider that, especially in Roman times, ancient historians tended to produce works at the behest of a sponsor. Assuming that political motivations may twist their objectivity is not a stretch.
Not only that but sometimes the works that have been preserved the best are obvious political tools. E.g. Julius Cesar's "The Gallic war".
Which, since we are on the topic, I absolutely must suggest as a reading for anyone who has the slightest interest in historical fact - or fiction - for that matter. Yes, the work is political boast about the campaign - but it is filled with juicy details on roman logistics, political action in the region, and so on.
I think this is why I enjoyed reading Montifiorre's Stalin biographies. Stalin was so universally hated in his native Georgia that there was no taboo in or desire to withhold all the details of his life that would have been omitted in a biography of a more respected leader.
Which, since we are on the topic, I absolutely must suggest as a reading for anyone who has the slightest interest in historical fact - or fiction - for that matter. Yes, the work is political boast about the campaign - but it is filled with juicy details on roman logistics, political action in the region, and so on.