Your first faction.

My first faction was Sylvan, and for a long time, it was my favorite, because in the third game, I played as Fortress, so in the fourth, I naturally chose Sylvan.

All parts of Heroes start with Humans, what is this, a conspiracy?

Well, it seems that this is correct; Humans are the most standard faction, and everyone played as them, so each part started with them to show how it has changed and how the whole game has changed! This is especially clearly seen in the fifth game.