Tidus realises, oh, she has a way of speaking, doesn't she? Mainly with the use of accuracy in regards to the roster (or her memory?), that makes him wonder if she thinks it could be wrong. But also, it's probably just some fancy way with words, a momentary thought that's put aside as he considers his own path.

"Mmm - let me look at the roster," he decides, fairly easily enough, and takes a step in its direction. "Unless you printed out pictures with a list? It'll be easier so I don't forget anyone."

Well, of the people he does know about, both on and off - he doesn't have the most trustworthy people. But Tidus does allow for her to answer if she does happen to have a print out, before he'll move for the ICP if not. You know, in case she just happens to carry around that kind of thing.