My understanding is that both books were written/compiled toward the end of her life. There may have been more help from her assistants, including finding materials in her previous writings and helping her integrate it into a coherent narrative. I think it’s Prophets and Kings that ended up being published after she died. So, besides her writing style evolving over time, there may have been some pressure on her and her team to get the manuscripts done—at a time when her health (and abilities) may have been beginning to fail. In short, it may have mattered who was helping her polish the final manuscripts.