So... why LISA?

At the beginning, we the audience see that MICHAEL perceives LISA as someone special because she looks and sounds different. There doesn't seem to be a logical reason as to why she's like that. It seemed to be just a surreal element (something common in Charlie Kaufman's movies), which makes sense within a movie's internal logic. Eventually, we find out that everyone looks and sounds the same only in MICHAEL's head. Looking back at the movie with this information in mind, it doesn't make much sense to me. Shouldn't he have gotten to know her even a little before his subconscious(?) started to "tell" him that she's different enough? That happens the second he sees and hears her for the first time. Why? She's a stranger.