Suppose we have a game about the Battle of Hastings. It's a game about a single day - 14 October 1066. Men on high ground holding fast against a sophisticated invading force, until their discipline breaks. The important factors are the advantages of those invaders, and the discipline of the defenders. When the game ends, one or the other would have won out. Though the end result might be wildly different from match to match, the essential nature of the battle remains intact, with the same troops fighting over the same ground, and each match will follow a broadly similar narrative arc.
Suppose we have a game about the invasions of 1066. It's a game about a single season - autumn 1066. Two invading armies prepare to cross the sea: the Norwegians under Harald Hardrada, and the Normans under William the (soon-to-be) Conqueror. England's King Harold Godwinson is caught in the middle. Historically, Harold prepared to oppose William's crossing, but when it was delayed due to bad weather, he disbanded his army and navy. This is when the Norwegians land up north, and Harold hightails it, defeating Hardrada in a bloody battle that costs him some of his best troops. He's still catching his breath when William lands, and the Battle of Hastings forces Harold to fight with what he can scrape together.
But it only happened that way because Hardrada landed before William did. What would the Battle of Hastings have looked like if William had landed first, and been opposed by Harold's housecarls? What if he never got to Hastings - what if his fleet had been sunk by Harold's? For that matter, what if the final battle hadn't been between Harold and William, but William and Hardrada? What if that first battle - wherever it was and whomever it was against - wasn't as decisive, resulting in a three-way war for control of England? Three-way wars don't remain three-ways for long - so, who throws in with who? Maybe William and Hardrada agree (for now) to split up the kingdom while combining forces against Harold? Maybe Harold throws in with Hardrada, making concessions to the Norwegian king and his ally, Harold's own brother Tostig?
Two given matches of this hypothetical game could play out very differently. The factions and their armies might be the same, but small changes can result in ripple effects, drastically altering not only the moment-to-moment game state, but also the final result of the contest.
Suppose we have a game about England in the eleventh century. It is a game about a hundred years, a series of invasions and rebellions, betrayals and victories. Who invades when, and how successfully, depends on other wars in other places - on conflicts in Normandy, Denmark, Norway. I would wager that the chances of the game ending with the Norman Conquests is pretty slender - it's just one of many possible outcomes of this complicated and intricate dance.The larger the scope of a wargame, the less likely it is that it will hue to the historical narrative. This isn't a fault of such games, nor necessarily a strength: it's simply a quality inherent to such games. Similarly, the tighter the scope of the game, the more it will conform to the beats of what actually happened, and the more plausible its counterfactuals will appear. One isn't better than the other, or even more accurate than the other. A tightly-scoped game might better capture how the thing went down and why, but a game with a broader scope can better capture how the thing felt, how history is lived in the moment, a mass of opportunism and accidents.