I know, it's somewhat hard to explain. The best way I know to point you towards the difference I see is considering a combination of scale and chance. The events I want to focus on should be small (not in the sense of their significance, but in the sense of their complexity) and easily affected by random variation. I don't know that there's a clear breaking point, and it's possible I'm overly affected by the writing of historians, who tend to tell the story of the past in such a way that certain events and outcomes are seen as "big deals" while others were inevitable steps or random chance. Since I know the history, I know what the "big deals" are and can steer around them and just feel like I'm not committing the sin of alternate history that annoys me.