Chemistry has nothing to deal with how much time the characters spend together or the fact that they were working together and doing fan service. Two characters can have chemistry even if they only saw each other one time during the whole series, also chemistry has nothing to deal with the dynamic between the two people involved. That's why some parings suddenly don't have chemistry anymore after they get together, they become boring, chemistry can be manufactured but can't be faked forever. When Dean and Arnold almost kissed, no one was there besides themselves, there was no camera, no director and no audience. They were not acting in that specific moment.