That was, indeed, on the drama CD that shipped with Sen no Kiseki's limited edition.
Good job at, eh, recognising patterns that are really quite obvious. As for plausible explanations behind them: for FC and SC Falcom were very much looking for a new format, largely sticking to what had worked in previous Legend of Heroes games (e.g. White Witch was also one nation per chapter). They already started drafting Zero by the time they worked on The 3rd, allowing Zero and Ao to become this meticulously crafted RPG that ended up exactly as they had initially planned over the course of four years. With Sen -- merely two years later already -- they had to make the transition to 3D, and decided to stick to FC/SC's systematic approach in order to introduce the enormous empire to the player. In the end, they ran into development problems and cut the final chapter, shipping Sen with an awkward ending and padding Sen II as the final chapter with additional content, and did a great job on that.