"Exactly what it is. Maybe it was just a slip of the mind sort of thing. I kind of wondered what happened to us when we get pulled here. Maybe the guy that I replaced wound up replacing me and you just didn't realize. I think the only reason I realized that you weren't Chuck is because you're you, and not someone else." Not that Phineas was really the creme de la creme of noticing the obvious, but he was certain that even if he were, he wouldn't have noticed it if it was anyone but his stepbrother. Or possibly if it were Godzilla.
"Let's get going, It's a little bit of a hike to the lecture hall."