After further research, I discovered something:the Ansem you fight in the first game isn't Ansem or Xehanort. It's Xehanort's Heartless, and he took Ansem's name because he didn't have one. This means that the old guy in BbS really is Xehanort, before he was split in two (Ansem and Xemnas)

By the way, anyone know what else you can spell if you rearrange "Xemnas"? :D