An important thing to think about it the definition of "history" here. In the academic study of history, "history" can be defined as either "events in the past" or "the written record of the past". The latter is much more common. History in the classics world is more of a literary genre than "the events". Classics is somewhat interdisciplinary, in that it deals with both history and literature. The big difference between history and classics is that classics tends to focus more on texts for themselves, hence the heavier focus on literary criticism. In the realm of historiography, for example, a modern historian will look at events and presuppositions that may be influencing the author of a primary source, as well as certain lenses through which to look at that primary source. Modern history relies a great deal on philosophy and sociology to aid in understanding primary sources. An ancient historian, when evaluating a primary source has many more things to take into consideration. Speeches recorded in ancient sources were often fabricated by the author of the source. By "fabricate" I do not mean that the author just wrote some random speech, I mean he did not recreate the speech word for word. Rather, he wrote a speech that was appropriate to the occasion. He would often imitate the style of the speaker, saying things that he believed would get the audience to react the way they did. With that being said, speeches can also be used to show dramatic irony or to foreshadow events, like Thucydides' Funeral Oration of Pericles. Occasionally, events can be used to mirror other events, or as microcosms of a larger event. Tacitus uses Nero to represent the emperor Domitian, with whom he was a contemporary. Thucydides uses the plague in Athens (which, by the way, there is archeological evidence for) as a microcosm for the war as a whole. In my own field, ancient military history, the most contested area is numbers of soldiers. In Greek battles, the enemy's numbers will often be inflated. To my knowledge, the Romans do this less. Additionally, geography may be abbreviated. A few sources I have drawn on here that might be helpful if you are interested. I particularly recommend :

A.J. Woodman, Rhetoric in Classical Historiography. Four Studies Croom Helm, 1988.

Frank Walbank, Polybius, Rome, and the Hellenistic World Cambridge Univeristy Press, 2002.

Robin Lane Fox, “Thucydides and Documentary History.” Classical Quarterly 60.1 (2010) 11-29.

Vivienne Gray, “Thucydides’ Source Citations: ‘It is Said’.” Classical Quarterly 61.1 (2011) 75-90.

David Gribble, “Narrator Interventions in Thucydides.” Journal of Hellenic Studies 118 (1998) 41-67.

James M. Scott, “The Rhetoric of Suppressed Speech: Tacitus’ Omission of Direct Discourse in his Annals as a Technique in Character Denigration” AHB 12.1 (1998) 8-18.