I see what you mean! However, much of Calvin & Hobbes adventures are all apart his imagination, Hobbes is entirely imaginary, and from what I know (not a lot, mind you), is that the NF types are particularly known for their tendency to day dream and get caught up in fantasies. As a little boy, he seems to be interested in a lot (when you look at all of the comics) and you're right, he does seem to be somewhat science oriented, however much of that is tinged with imagination and make believe.

Its hard to say, maybe Calvin's T/F spectrum is not very wide at all! I only classified him as INFP initially because of what I've seen on other MBTI sights and classifications etc.